As long as there is 'nada', there is the mind, because it is the mind that perceives the sound.
The nada (sound), arising from Pranava and Brahma ("fore-sound", referring to Om) has the nature of light.
Nadabindu & Dhyanabindu Upanishads. Swami Satyadharma Saraswati
Sound has tremendous power to influence our consciousness. We all know how sound affects our moods and emotions, and some of us are able to fall asleep to the sound of television only. However, there are ways to interact with sound in a more constructive way.
Nada Yoga is a healing form of meditation (in this case, concentrating or focusing on sound) practiced since time immemorial by yogis who lived in the wild: in jungles, forests, mountains, on the banks of rivers, seas and oceans. They simply (and, at the same time, not easily) listened to the sounds of nature! That is why we should listen to the wind blowing, birds singing, the murmur of a brook, the sound of waves or a waterfall.
How it works: concentrating on an external sound, even a loud one, our mind, diving after it, learns to distinguish in the loud one a quieter and more subtle sound, gradually sinking into deeper layers of consciousness.
Whatever external sound initially enters the field of your attention, "stick", listen to it, excluding all other sounds, as if you were concentrating on looking at a symbol in a picture; and then after a while this sound will absorb your attention entirely.
A regular 15-30 minute practice is to listen to a single sound and keep your attention on it, reaching a state of complete absorption. The mind should not jump from one sound to another. Gradually in the louder one, you will learn to distinguish the quieter, then barely audible and once even that inaudible inner sound, which in the ordinary state is not perceivable either through the senses or by the mind.
In the ordinary state, our mind, left to itself, like a frisky squirrel hopping from branch to branch, instantly jumps from one thought to another, from one desire to another. Nada Yoga allows us to heal from this "weakness" by calming and slowing the mind and eventually stopping the flow of thoughts.
The basis of sound is vibration, and vibration is the basis of the universe or creation. Infinite space (Akasa) is the first element of creation, so absorption in sound leads to the experience of cidakasa – absorption of the mind in the dimension of space – when all external phenomena, all illusory experience of the external world, disappears..