We Christians know God's voice when we hear it. He authenticates His own words:
And when he putteth forth his own sheep, he goeth before them, and the sheep follow him: for they know his voice. And a stranger will they not follow, but will flee from him: for they know not the voice of strangers. – John 10:4-5 My sheep hear my voice, and I know them, and they follow me: - John 10:27
But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know [them], because they are spiritually discerned. - I Cor 2:15
Those without “ears to hear” cannot hear Him, they don’t know the difference between the words of God and the words of men. In the following passage, the people were physically hearing Jesus directly (sound or pressure waves) - but they could not spiritually hear Him:
Why do ye not understand my speech? [even] because ye cannot hear my word. – John 8:43
Signs and reason will not do, one must be given the gift of “ears to hear:”
And Moses called unto all Israel, and said unto them, Ye have seen all that the LORD did before your eyes in the land of Egypt unto Pharaoh, and unto all his servants, and unto all his land; The great temptations which thine eyes have seen, the signs, and those great miracles: Yet the LORD hath not given you an heart to perceive, and eyes to see, and ears to hear, unto this day. And I have led you forty years in the wilderness: your clothes are not waxen old upon you, and thy shoe is not waxen old upon thy foot. - Deuteronomy 29:2-5
Again, God’s words are not like the words of mere mortals. For one thing, they are creative which means He cannot lie. When He says a thing, it is. It is because He said it. He said "Let there be light" and there was light.
For he spake, and it was done; he commanded, and it stood fast. - Psalms 33:9
Moreover, His words are not dead letters like the words of men. They are alive, they enliven us.
But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God. – Matt 4:4 Give us this day our daily bread. – Matt 6:11
I am that bread of life. – John 6:48
It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, [they] are spirit, and [they] are life. - John 6:63
God preserves His own words.
The words of the LORD are pure words: as silver tried in a furnace of earth, purified seven times. Thou shalt keep them, O LORD, thou shalt preserve them from this generation for ever. - Psalms 12:6-7
The mortals He chooses to be scribes, compilers, keepers or even speakers of His words are every bit as irrelevant as the media by which He conveys His words. He has even used pagans and non-believers to declare what He wanted said:
Nebuchadnezzar the king, unto all people, nations, and languages, that dwell in all the earth; Peace be multiplied unto you. I thought it good to shew the signs and wonders that the high God hath wrought toward me.
How great [are] his signs! and how mighty [are] his wonders! his kingdom [is] an everlasting kingdom, and his dominion [is] from generation to generation. – Daniel 4:1-3
Give GOD the glory, not man, never man. And upon a set day Herod, arrayed in royal apparel, sat upon his throne, and made an oration unto them. And the people gave a shout, [saying, It is] the voice of a god, and not of a man. And immediately the angel of the Lord smote him, because he gave not God the glory: and he was eaten of worms, and gave up the ghost. But the word of God grew and multiplied. – Acts 12:21-24
His Name is I AM.