In the beginning, there was sound, the word, and the sound was with God. You know, amongst different manifestations, the manifestation of sound is the subtlest and that is why the ear is considered as the subtlest organ. Now, these acoustic expressions are of two kinds: one is the divine, or spiritual acoustic expression and the other is the physical acoustic expression. The sound you hear is physical acoustic expression and the divine is not perceived by the ordinary human ears. But there is the vaster spiritual acoustic expression in the divine realm. But in the case of Nirguna Brahma, there is no expression, because Nirguna means “with out quality or expression.” But even in the case of Saguna Brahma, the attributional Entity and even starting from Purusottama, the nucleus human consciousness, there is expression, manifestation. And that expression, till it reaches the physical sphere, is spiritual acoustic expression not amenable to human ears.
Whenever you think, you create a mental sound. What is thinking? Thinking is mental speech. The sound of silence. And it is what is known as omkara in Sanskrit when those inner senses develop, then in the first phase sadhakas can hear that that divine sound. In the first phase it is like the sound of crickets.
Then in the next phase, as if somebody is dancing with ghunghur (ankle bells). Next you will hear the sound of flutes. Then the sound of the ocean, you know? A particular sound is created by the sea. And then, in the fifth phase, the sound of bells. And finally, the sound is just like oonnn the omkara in pure form. And after that there is no sound, because after that there ends the realm of Saguna. After that there is the scope of Nirguna. In the realm of Nirguna there cannot be any sound, because there cannot be any manifestation. Not even divine expression, not even supra-psychic expression.