Here we see Aswapati travelling through the subliminal layers of consciousness through the sound. These experiences of an inner hearing are well-known to yogis and come as our inner being opens to higher states of consciousness.
At the Feet of The Mother
Canto XIV: The World-Soul
Aswapati reached the Self of Mind but the Soul of the world and its mystic heart was yet unseen. Now he suddenly comes across a hidden passage, a well or a tunnel through which he can go further.
Aswapati in his search for the master-key, that would uplift earth-nature towards the divinity hidden deep within it, has arrived at some vast impersonal state. He enters the Cosmic Consciousness.