According to Advaita, there is only Brahman, and nothing more. Everything you see, the world, is just Brahman.
Vivekacudamani, Sri Adhishankaracharya - verse 231
brahm'aiv'edaṁ viśvam ity'eva vāṇī
śrautī brūte'atharva-niṣṭhā variṣṭhā
tasmad etad brahma-mātraṁ hi viśvaṁ
The scriture [atharva veda] says this world is Brahman. All is
Brahman. Therefore, all this world is nothing but Brahman. What you
impute to a thing [superimpose] is not separate from that to which you
impute [the substratum]. The basis of all is Brahman. Therefore, what
you call the world is not the world, but Brahman.
So, Sri Krishna and everyone else are Brahman. When Sri Krishna speaks is only Brahman who speaks. But also when a little ant die, it is just Brahman who die.
yac cā'pi sarvabhūtānāṁ
bījaṁ tad aham arjuna
na tad asti vinā yat syān
mayā bhūtaṁ carācaram
And further, whatsoever is the seed of all existences that am I, O
Arjuna; nor is there anything, moving or unmoving that can exist
When you see Brahman as a little insect with 6 legs, etc, you call it "ant Brahman" (usually without the Brahman part, just ant ;) ). When you can put qualities to something, that is saguna. And as Brahman is everything, everything in manifestation is saguna Brahman.
But, in order to realize that, we pay special attention to the "sattva-saguna-Brahman part", which are sages, jivanmuktas, avataras, devas, srutis, shanti, cit, etc. And we usually call saguna Brahman only to that part (and leave ant, and everything else, out of the term, only to help understanding and realization).
Vivekacudamani, Sri Adhishankaracharya - verse 117
sattvaṁ viśuddhaṁ jalavat tath'āpi
tābhyāṁ militvā saraṇāya kalpate
yatr'ātma-bimbaḥ prati-bimbitaḥ san
prakāśayaty arka iv'ākhilaṁ jaḍam
Sattva is pure. It becomes useful for liberation. Therein is reflected
the shadow of the Atman. Sattva manifests the Atman, as the sun
manifests the whole of the universe. It is light. Light (sattva),
scattering (rajas), and darkness (tamas) are the three qualities
When you think about Brahman as without upadhis only (without maya), without any quality that you can think of, that kind of sunya (void, which actually is not void because is full of Brahman), that you call it nirguna Brahman. So, nirguna Brahman cannot speak, because if He does, then we call it saguna Brahman.
But, as Brahman alone is, He is the only one who speaks, and listen and does everything.
brahmā'rpaṇaṁ brahma havir
brahmāgnau brahmaṇā hutam
brahmai'va tena gantavyaṁ
For him the act of offering is Brahman, the oblation is Brahman. By
Brahman is it offered into the fire of Brahman. Brahman is that which
is to be attained by him who realizes Brahman in his works.
Advaitin theory is not that Brahman is without any characteristic, but Brahman is all, and not something in special to exclusion of other parts. There is no two Brahman (Saguna-Nirguna), there is only one, and names are only to understand Brahman, so Brahman is with and without gunas (gunatita Brahman, beyond gunas, with and without)
Vivekacudamani, Sri Adhishankaracharya - verse 243,244
tayor-virodho'ayam upādhi kalpito
na vāstavaḥ kaścid upādhireṣaḥ
īśasya māyā mahad-ādi-kāraṇaṁ
jīvasya kāryaṁ śṛṇu pañcakośam
etāv upādhī para-jīvayos tayoḥ
samyaṅ nirāse na paro na jīvah
rājyaṁ narendrasya bhaṭasya kheṭakaḥ
tayor apohe na bhaṭo na rājā
Though they are of different natures (jiva and Brahman), yet in their
proper essence, they are equal. What difference you see is only in
upadhi, or qualifications (gunas). The upadhi of Brahman is Maya which
is the cause of Mahat and its works; and the upadhi of jiva is the
five koshas (bodies) and their works. Both are one substance. This
substance plus Maya and Mahat is Brahman, and the same substance plus
the five koshas is the jiva. Take away the upadhis of both, and what is
left is the same thing....