You quoted from the Mahabharata this passage of text:
Meanwhile, the righteous-souled Suka, who had entered the
elements, had become their soul and acquired omnipresence,
answered his sire by uttering the monosyllable Bho in the form of an echo. At this, the entire universe of mobile and immobile
creatures, uttering the monosyllable Bho, echoed the answer of
Suka. From that time to this, when sounds are uttered in
mountain-caves or on mountain-breasts, the latter, as if in answer to
Suka still echo them (with the monosyllable Bho). Having cast off all
the attributes of sound, etc., and showing his Yoga-prowess in the
manner of his disappearance, Suka in this way attained to the
highest station.
I think that Vaishnavas would say that the Mahabharata passage which you quoted doesn't support Advaita system of Vedanta by Shankara because of the following reasons:
One reason is that the quoted text says that Suka "had become their soul" which is like saying that he had become paramatma (Supersoul, Supreme self, Brahman) and the text also says he "acquired omnipresence" as if he had become omnipresent like paramatma/Brahman, but after that the text also says that Suka "answered his sire by uttering the monosyllable Bho". And the text also says "Suka in this way attained to the highest station" which also indicates that he acquired the highest possible state, namely the state of Brahman.
According to the teaching of the Advaita of Shankara when a jiva soul (jivatma) acquires the state of nirguna Brahman he loses his individuality as a jivatma and he doesn't exist as an individual anymore, therefore it would be impossible for him to utter any sound. Per Shankara's understanding of Vedanta, nirguna Brahman which is the highest station (the highest state of existence) is not an individual, he doesn't do anything, he has no any characteristics or qualities, and he cannot say or utter anything. Thus it seems that the quoted Mahabharata text doesn't support the Advaita of Shankara at all.
There are many other verses in the scriptures that are similar in character to the above quoted Mahabharata text. Here are some of those:
The Brihadaranyaka Upanishad 1.4.10 says:
Verily in the beginning this was Brahman, that Brahman knew (its) Self
only, saying, 'I am Brahman.' From it all this sprang. Thus, whatever
Deva was awakened (so as to know Brahman), he indeed became that
(Brahman); and the same with Rishis and men. The Rishi Vâmadeva saw
and understood it, singing, 'I was Manu (moon), I was the sun.'
Therefore now also he who thus knows that he is Brahman (another
translator: "I am Brahman"), becomes all this, and even the Devas
cannot prevent it, for he himself is their Self (another
translator: for he becomes their self).
In this Brihadaranyaka Upanishad passage we see that sage Vamadeva realised himself to be "I am Brahman", and also that he had become "their self", namely it is as if the text says sage Vamadeva is Brahman or had become paramatma ("for he becomes their self").
Mundaka Upanishad 3.2.9 says:
sa yo ha vai tat paramaṃ brahma veda brahmaiva bhavati
"He who
knows that highest Brahman, becomes even Brahman."
Here it is said that the jivatma becomes Brahman.
Some verses in the scriptures even say that jivatma will become Vishnu. Here are some of those examples:
Narayana Upanishad says:
http://www.sacred-texts.com/hin/tmu/tmu20.htm
ya evaṃ veda sa viṣṇureva bhavati
yaḥ = whosoever; evaṃ = this; veda = knows; saḥ = he; viṣṇuḥ = Vishnu,
the all-pervader; eva = indeed, surely; bhavati = becomes
"Whoever knows Him thus, becomes Vishṇu Himself."
Padma Purana, Uttarakhanda 71.29 says about the person who is uttering the names of Lord Vishnu:
He, by reciting, muttering, meditating upon Vishnu's name in any
manner, is freed. There is no doubt that he would be Vishnu
himself.
Nrisimha Purva Tapani Upanishad, chapter 1.7, translated by Paul Deussen, Sixty Upanisads of the Veda, vol ii, p. 818:
https://books.google.co.in/books?id=XYepeIGUY0gC&printsec=frontcover#v=onepage&q&f=false
And one who knows this great Upanisad, he becomes, when he has
performed the preparatory worship, the great Vishnu, the great
Vishnu.
It is well known that Vaishnavas and the scriptures likewise identify Lord Vishnu as the Supreme or Brahman (*1). So someone may try to interpret those statements which I quoted above saying "he becomes Vishnu" as if they say that jivatma will become Lord Vishnu, or alternatively if word "vishnu" is taken to mean "All-pervader" as denoting not God Vishnu but Brahman who is said to be omnipresent in the scriptures, then Vaishnavas would say that both of these interpretations are not possible.
It is not possible for a jivatma to become Lord Vishnu simply because He is one god (*1) who eternally (*2) holds that position, and therefore no one can replace him and take over his position.
And that other interpretation according to which a jivatma is supposed to become "all-pervader" also makes no sense because the jivatma is said to be atomic in size (*3), and also is said to be unchangeable.
It is stated in the Bhagavad gita 2.25 that the soul is avyakto ’yam acintyo ’yam avikāryo ’yam ucyate, where the word avikāryo mean "unchangeable", it means that the soul cannot change its size because if it changes its size it would not be avikārya.
So it makes no sense to think that the jivatma can change its size from being small atomic particle to become huge soul "all-pervading" everything, ie to become literally omnipresent in its size.
Also it's worth to mention that the scriptures say the jivatma is eternal (*4), so he cannot cease to exist (*5). The jivatma is eternally small atomic particle of spirit, and it stays to be like that always, never changing its size. Even when he gets moksha he will stay to be such forever.
The above Brihadaranyaka Upanishad 1.4.10 and Mundaka Upanishad 3.2.9 verses also seem not to support Advaita of Shankara just like the above Mahabharata text. And this is so because per the Shankara's Advaita we already are Brahman and thus it makes no sense to say that we will "become" Brahman as these verses say. Actually when I think about these verses it sounds to me as if they are written according to some Vaishnava system of Vedanta. This is so because it is possible to "become Brahman" in Vaishnava systems of Vedanta and stay to be an individual (jivatma) even when you become Brahman/paramatma/Lord Vishnu. Of course, to "become Brahman" in Vaishnava understanding doesn't mean to lose individuality and to become literally the Supreme. The expression "become Brahman" Vaishnavas interpret in a specific manner.
Vaishnavas think that scriptural statements of that kind, saying something like "he will become Brahman" or "he will become the inner self of all (paramatma)", simply means that the jivatma will enter into Brahman like a fish is thrown into the ocean, and thus the fish will "become one with" the ocean, ie he will stay to be a small atomic part of an huge ocean. So in this sense a fish is the ocean when he enters into it. It's not that a fish is a whole ocean. And also the fish doesn't lose his individuality (jivahood, to be a jivatma) in the ocean. In this sense the statements "he will become Brahman/paramatma" or even "he becomes Vishnu" and "I am Brahman" should be understood. It's like a fish would say "I will become an ocean" when thrown into it, and when he finally enters an ocean the fish can say "I am the ocean", which is equivalent to the statement "I am Brahman".
There is a nice example of this in the Mundaka Upanishad 2.2.4:
Om is the bow, the Self (jivatma) is the arrow, Brahman is
called its aim. It is to be hit by a man who is not
thoughtless; and then, as the arrow (becomes one with the target), he will become one with Brahman.
This is similar to the above example I said about the fish which is thrown into the ocean. Here the jivatma is like an arrow which hits the target (Brahman), and thus literally enters the target (Brahman), and stays like that in the target, ie in Brahman. Thus even when an arrow enters the target, it stays in it and the difference between the arrow and target still exist although we could say that they have become one, we could say "an arrow had become the target". Similarly we say for the fish or the jivatma, "he becomes an ocean" or "he becomes Brahman".(*6)
Thus the individuality of the jivatma will continue to exist even when the jivatma enters into the pure existence of Brahman. Just like an arrow will continue to exist after it hits the target, so the jivatma likewise. To attain Brahman doesn't mean to lose individuality.
So the conclusion:
It's more likely those scriptural statements support Vaishnava systems of Vedanta, because in them you can "become Brahman" like a fish can become one with the ocean, like an arrow can become one with the target, and still you retain your individuality, as I explained.
Footnotes from the above post:
-- (*1) Narayana Upanishad says: "Nārāyaṇa who pervades all elements, who is one only, who is the cause Purusha and who is causeless, is known as Parabrahman."
So, the verse says there is only one Lord Narayana, namely there are not two, three, or more gods Narayana. He is called Brahman or Parabrahman (Supreme Brahman) in this verse also. For this reason it makes no sense to think that jivatma can become yet another Lord or God Narayana. Thus statements that I quoted above such as "Whoever knows Him thus, becomes Vishṇu Himself." cannot be interpreted to mean that jivatma will become Lord Vishnu (Narayana), but should be interpreted in the sense as I explained, namely it means that the jivatma can become a resident (inhabitant) living in Vishnu or Brahman when he gets moksha and enters into Brahman, like a fish which enters an ocean can become a resident (inhabitant) of an ocean, and thus living in the ocean a fish can say "I have become an ocean".
-- (*2) Narayana Upanishad says: "Nārāyaṇa is eternal".
See also Narayana Sukta of the Yajur Veda describes Lord Narayana:
viśvataḥ paramān-nityaṃ "He is the greatest in the universe (viśvataḥ paramān), eternal (nityaṃ)",
and also patiṃ viśvasyātmeśvaragṃ śāśvatagṃ "He is the Lord of all worlds (patiṃ viśvasya), Lord of all beings (ātmeśvara), eternal (śāśvata)".
-- (*3) There are several ways how atomic size of the jiva soul (jivatma) can be proved. Here are some of those:
a) Mundaka Upanishad 3.1.9 says: eṣo ’ṇur ātmā "The soul is atomic in size".
The word aṇu means "atomic".
b) Svetasvatara Upanishad 5.9 says:
bālāgra-śata-bhāgasya śatadhā kalpitasya ca bhāgo jīvaḥ sa vijñeyaḥ sa cānantyāya kalpate
"When the upper point of a hair is divided into one hundred parts and again each of such parts is further divided into one hundred parts, each such part is the measurement of the dimension of the spirit soul."
Thus the size of the jivatma is said to be like a 10000th part of the tip of a hair in this verse, which is very small size.
By the way, this verse ends with the words sa cānantyāya kalpate which some translators used to translate "and yet it is to be infinite" because they think the word ānantya means "infinite". However such translation makes no sense because of two reasons. One reason is that if we assume ānantya means "infinite" it would directly violate that what is said in the first part of the verse, namely that the size of the jivatma is as small as 10000th part of the tip of a hair. And the other reason is that it would violate all the other verses that I quoted in which the size of the jiva soul is established to be of very small size and not infinite. According to the Sanskrit dictionary ānantya means "infinite", but also "immortality" and even "future happiness". Thus instead of "infinite" an appropriate translation should be that the soul is fit to achieve immortality and future happiness, which is consistent with the verse I also quoted in this post from the Svetasvatara Upanishad 6.12 "to them belongs eternal happiness" (see below footnote 4c).
c) Mundaka Upanishad 2.1.1 says: "This is the truth. As from a blazing fire sparks, being like unto fire, fly forth a thousandfold, thus are various beings brought forth from the Imperishable, my friend, and return thither also."
In this verse Brahman is compared to huge fire, and jiva souls (jivatmas) are compared to small sparks of that huge fire. The spark of fire is very small particle of fire, and thus the verse claims that the jivatmas are small in comparison to huge omnipresent Brahman. That verse also says that the jivatmas are in quality the same as Brahman called "the Imperishable" in the verse, which means that the jivatmas are essentially the same as Brahman, ie they are like Brahman in their essential nature because they are compared to sparks which are in essence just like a fire ("sparks, being like unto fire") but are small particles of that fire. So, jivatmas are small particles of Brahman according to this verse, namely they are not huge omnipresent soul. The verse also says that there are many such small sparks ("fly forth a thousandfold") which means that there are many souls (jivatmas) in the world, namely the theory of some interpreters of Vedanta who say that there is just one soul in the world is not supported in this verse.
d) Katha Upanishad chapter 6, verse 16 says: "There are a hundred and one arteries of the heart, one of them penetrates the crown of the head. Moving upwards by it, a man (at his death) reaches the Immortal; the other arteries serve for departing in different directions."
In this verse it is explained how the jivatma is going out of the body at death. The jivatma is located in the heart of the body. And when a man dies the jivatma have to start moving from the heart through one of those arteries (nadis), as the verse says "There are a hundred and one arteries of the heart", and the verse says that the jivatma can depart from the body passing through an artery that ends at the crown of the head. So the jivatma must go through an artery from the heart to the head. It indicates that the jivatma is small in size because if it's not small it would be impossible to go through an artery and travel like that. By the way, if we are not the small jivatma but the all-pervading omnipresent soul then there would be no need to travel through some artery and reach the next body for rebirth. The omnipresent soul could just somehow appear in the next body, there would be no need to travel to reach that body. A soul which is so much huge that is omnipresent, is located everywhere in the universe pervading this earth, heaven, whole universe and even beyond this universe. For such a soul there is no purpose to travel anywhere because that soul already is located everywhere. However, we see that the scriptures say every time when a person dies, he has to travel first through an artery (nadi) to leave the body, and then far distances to reach the next body for rebirth. That strongly suggests that jivatma soul is small in size, namely atomic in size, as we also see from other scriptural examples which I'm quoting here.
e) Brihadaranyaka Upanishad 4.3.38 says: "And as policemen, magistrates, equerries, and governors gather round a king who is departing, thus do all the senses (prânas) gather round the Self at the time of death, when a man is thus going to expire."
In this verse it is said that the senses (pranas) gather round the Self (jivatma) at the time of death. This is significant because it says the senses "gather round the Self (jivatma)". If the self is huge, omnipresent, located everywhere, pervading the whole universe, it would be impossible for that self to be surrounded by anything. For it make no sense to say that such a huge self can be surrounded by the senses in the body. Only a small object can be surrounded by something such as the senses in the body. For example, we can say that a tree is surrounded by other trees in the woods. But if we talk about a tree which is so much huge that it is bigger than the woods, then it would make no sense to say that such a tree can be surrounded by anything in the woods. So the self referred to in the verse must be a small object, a small self, and it cannot be the omnipresent all-pervading self. Only then the verse makes sense, only then it can be said that "the senses (prânas) gather round the Self". The verse also says that at death the Self (jivatma) is departing like a king who is departing. I have already explained in point "d)" that departing or traveling to somewhere makes sense only for the small soul, and not for the omnipresent soul for there is no purpose for the omnipresent soul to travel anywhere. And a small soul has to travel because only by traveling he can reach the body at the far distance to take the new rebirth.
All the above verses from a) to e) support the view that the jivatma is small in size and not omnipresent.
-- (*4) There are many ways how to prove that, here are just some examples:
a) Bhagavad gita 15.7: mamaivāṁśo jīva-loke jīva-bhūtaḥ sanātanaḥ "The living entities in this conditioned world are My eternal fragmental parts."
In this verse Lord Krishna clearly says that living entities (jivas) are eternal (sanātanaḥ). So individuality of the jivas is never destroyed, got lost or cease to exist.
b) It is said in the Svetasvatara Upanishad 6.13:
nityo nityānāṃ cetanaś cetanānām eko bahūnāṃ yo vidadhāti kāmān
"He (The Supreme Lord) is the supreme eternal among all eternals and the supreme conscious entity among all conscious entities, who fulfills everyone's desires and needs."
In this verse living beings (jivas) are called "nityānāṃ" which means "eternals" and The Supreme Lord (paramatma) is called "nityo" which means "One eternal". Thus we see that eternal difference between the two types of souls -- jivatmas and paramatma -- is established in Sruti scriptures.
c) Svetasvatara Upanishad 6.12 says: "The wise who perceive Him (the one God, the one ruler of many) within their self, to them belongs eternal happiness, not to others."
This verse describes the state of moksha in which the liberated soul will have the experience of (or feeling of) eternal happiness (happiness will never stop). Thus the verse tells that the individuality of jivatma will continue in moksha eternally because it says the jivatma will feel "eternal happiness" (sukhaṃ śāśvataṃ), and thus jivatma will never cease to exist. The jivatma exists eternally, and in moksha he will feel happiness eternally. This is the meaning of the verse.
d) Brihadaranyaka Upanishad 4.3.30 says "For knowing is inseparable from the knower, because it cannot perish."
In this verse an individual soul (jivatma) is called "the knower" because he has ability to know something, to be aware of something. From this statement we learn that the jivatma stays to be an individual for whole eternity, and also that jivatma's ability to know something (called as "knowing" in the verse), to learn or to be aware of something, also exists eternally. That eternality is expressed with the words "it cannot perish", ie it cannot die, vanish, fade away. So neither the knower (an individual jivatma) will perish, nor his knowing (his ability to know something) will perish. Or that is to say both of them, the knower and his knowing, will never cease to exist.
-- (*5) Bhagavad gita 2.12: "Never was there a time when I did not exist, nor you, nor all these kings; nor in the future shall any of us cease to be."
-- (*6) Brihadaranyaka Upanishad 4.3.32 says: salila eko draṣṭādvaito bhavati | eṣa brahmalokaḥ samrāṭ | iti hainam anuśaśāsa yājñavalkyaḥ | eṣāsya paramā gatiḥ | eṣāsya paramā saṃpat | eṣo 'sya paramo lokaḥ | eṣo 'sya parama ānandaḥ | etasyaivānandasyānyāni bhūtāni mātrām upajīvanti
"An ocean becomes that seer (jivatma), without any duality; this is the Brahma-world, O King. Thus did Yâjñavalkya teach him. This is his highest goal, this is his highest success (or wealth), this is his highest world, this is his highest bliss (ānanda). All other creatures live on a small portion of that bliss."
This verse illustrates the point that a self-realised sage or jivatma becomes one with Brahman just like, say, a fish becomes one with the ocean, or just like an arrow becomes one with the target. That type of unity is especially seen in the verse to be expressed with the words "without any duality", which simply means that there is no more duality or separate existence between a released jivatma and Brahman, just like there is no more duality or separate existence between a fish who lives in the ocean and the ocean itself, or just like there is no more duality or separate existence between an arrow which is in the target and the target itself because they have become one, that is, they are in unity without duality.
