A sound wave is the pattern of disturbance caused by the movement of energy traveling through a medium

A sound wave is the pattern of disturbance caused by the movement of energy traveling through a medium (such as air, water, or any other liquid or solid matter) as it propagates away from the source of the sound. The source is some object that causes a vibration, such as a ringing telephone, or a person’s vocal chords.
define medium-
A substance that makes possible the transfer of energy from one location to another, especially through waves. For example, matter of sufficient density can be a medium for sound waves, which transfer mechanical energy. See more at wave.
what is an observer in sound waves?
Observer is a special person (or a system that contains such person) which does not obey the usual laws of quantum mechanics. While it is much easier to define observer from a philosophical point of view, the mathematical answer is that the observer is a system which manifests subjective decoherence when observed