चत्वारि वाक् परिमिता पदानि तानि विदुर्व्राह्मणा ये मनीषिण: |
विदुर्व्राह्मणा ये मनीषिण: गुहा त्रीणि निहिता नेङ्गयन्ति तुरीयं वाचो
मनुष्या वदन्ति ॥ R.V. 1.164.45 ||
Four are the definite grades of speech: those Brahmans who are wise
know them : three deposited in secret, indicate no meaning: men speak
the fourth grade of speech. (H.H.Wilson).
Para, Pashyanti, Madhyama and Vaikhari ( परा , पश्यंती , मध्यमा , वैखरी ) are the four successive phases from which sounds pass through before we can hear it or it becomes audible. These are four successive stages of transformations of sound. By these four phases, we can learn about the origination or utterance of a word from the mouth. In Sanskrit, these are called Vani or vaak.
Among these, the three are hidden inside our body but one can experience the fourth i.e Vaikhari speech. First, let's take a quick look at what are they.
Para - eternal, Pashyanti - felt by a sage or a person in his deep consciousness, Madhyama- when it translates as an idea in the intellect, Vaikhari when the Vani is actually verbally expressed through the mouth, The daily spoken language.
The successive phases of Vaak Para, Pashyanti, Madhyama and Vaikhari are explained in Shrimad Bhagavata Purana
as follows - :
स एष जीवो विवरप्रसूति: प्राणेन घोषेण गुहां प्रविष्ट: ।
सूक्ष्ममुपेत्य रूपं मात्रा स्वरो वर्ण इति स्थविष्ठ: ॥ SB 11.12.17॥
sa eṣa jīvo vivara-prasūtiḥ prāṇena ghoṣeṇa guhāṁ praviṣṭaḥ
mano-mayaṁ sūkṣmam upetya rūpaṁ mātrā svaro varṇa iti sthaviṣṭhaḥ
This is that perceptible Supreme Lord who infuses life in all and who
manifests himself within the nerve -centres or plexuses (chakras)
known as Muladhara and others (existing in the internal parts of the
human body ). With the prana impregnated with nada (called para
speech), he enters the cave (known as adhara chakra ,located near the
anus). He proceeds (ahead) assuming the subtle mental form ( known as
pashyanti and Madhyama forms of speech in the spiritual plexuses
called Manipura and Vishuddhi located at naval and at the throat
respectively. He reveals Himself in the mouth in the form of short and
long notes, accents ( such as udatta,anudatta and svarita ) and
articulate sounds (like the velars , palatals ,dentals etc. ) This is
the grossest speech-form (called Vaikhari) of the Vedas and branches.
The meaning of the word Para is highest and in the context of sound, It is the highest eternal sound. So at the very beginning sound comes from an eternal source, which is always present in the Ether. In Hinduism we call it as a Shabda Brahman or Naad Brahma - ( शब्द ब्रह्म, नाद ब्रह्म ). Para Vani is divine which can be heard in the void (Shunyata) state of mind.
The Muladhara Chakra is the place of this Para form of sound from where it originates and reaches to the heart. This is described in following shlokas by author Bhartrhari in his work Vakyapadiya.
विशेषरहिता चेतनमिश्रा सृष्ट्युपयोगिनी जगदुपादानभूता
कुण्डलिनीरूपेण प्राणिनां मूलाधारे वर्तते
varṇādi -viśeṣarahitā cetanamiśrā sṛṣṭyupayoginī
kuṇḍalinīrūpeṇa prāṇināṃ mūlādhāre vartate
Without any speciality and with the consciousness Para in the form of
Kundalini resides at, originates at the Mulaadhaaracakra where it is
कुण्डलिन्याः प्राणवायुसंयोगे परा व्यज्यते
kuṇḍalinyāḥ prāṇavāyusaṃyoge parā vyajyate
Para form of sound manifests with the interaction of Prana-Vayu (vital
breath) with Kundalini.
So in the beginning sound starts its journey as constituted of air or vital breath as first stirring of the breath mixed with Kundalini Shakti which is conscious power inside us. We can call it as a chetana shakti also. It is said that Ishita Siddhi is required to master the Para speech.
After the first stage when this Para vani reaces to heart is called as pashyanti. Manipura Chakra is the seat of this vani. In other words, the language spoken from the heart is called Pashyanti, the faint whispering at the heart.
It is said that the ancient sages used to give the boons and curses with this type of speech
The next stage is called Madhyama is the intermediate stage.It is the speech which comes after some consideration and action done thoughtfully. It is in better words can be described as the speech which is active in the form of contemplation. Madhyama resides in our mind and in intellect. It is believed that the seat of this speech is Anahata chakra.
The Pashyanti Vak thereafter transforms into an internal
(antahs-amnivesini), subtle (sukshma) intellectual process (Jnana),
the level of thought (buddhi-matropadana), during which the speaker
becomes aware (parigrihita) of the word as it arises and takes a form
That sequence of thoughts results in a definite and clear array of
words. This is the intermediate stage – The Madhyamā vak, a sequenced
but a pre-vocal thought –described as the voice of silence; perhaps
best understood as internal speaking. Here, there is no perceptible
sound (Nada). The Madhyamā vak is in an inaudible wave or vibratory
Vaikhari : This is best explained as a daily spoken language, which do not require much thought. The faculty of speech. Vaikhari is articulate and according to commentary is of the lowest class.
The forms of Vaikhari speech are velars, palatals, dental etc i.e. matras , swaras , varnas ( मात्रा , स्वर , वर्ण ) in grammar. Unlike Para, Pashyanti, Madhyama other people can hear the Vaikhari speech. It is said that Prakriti and Paramatma both manifests through Vaikhari. Vaikhari is a full transformation of Vaak from subtle to gross form.
All the vaikhari sounds are considered as the voice of Brahman only.
A well-known scholar S.D.Satavalekar is explaining the commentary of the mantra you quoted Rig-Veda Subodh Bhashya
(a commentary on Rig Veda) in Hindi.
And finally here is a shloka mentioning the transformation of vaak from Yoga-Kundalini Upanishad
परायामंकुरी भूय पश्यन्तां द्वीदलीकृता।
मध्यमायां मुकुलिता वैखर्या
पूर्व यथोदिता वाग्विलोमेनास्तगा भवेत। || 3.18-19 ||
That vaak (power of speech) which sprouts in Para. Gives forth two
leaves in Pashyanti :buds forth in Madhyama and blossoms in Vaikhari
-that vak which has before been described reaches the stage of absorption of sound reversing the above order ( viz . begining with
Vaikhari , etc. ).
And here is next interesting shloka about Vaak from the same book.
20 - Whoever thinks that He who is the great lord of that vāk, who is
the undifferentiated and who is the illuminator of that vāk is Self;
whoever thinks over thus, is never affected by words, high or low (or
good or bad).