What is sound, and how do we hear it? Sound is scientifically defined as any vibratory disturbance in the pressure and density of a medium (solid, liquid, or gas) that stimulates the sense of hearing.
When molecules in gases are moved - by someone speaking, a pebble dropping into a pond, a hammer pounding a nail, or voices joinied in chant - energy is transferred, and the molecules begin to tremble and collide. As they bump together, each shimmering molecule passes energy along to its neighbors, carrying this vibration through the air (or other medium) in the form of a three-dimentional wave.
We take our ability to hear sounds for granted, but it's actually a wild and complex process. something happens out there to start air molecules vibrating, forming a wave of sound. Let's imagine for a moment that you are this sound wave. Air molecules crash into each other like billiard balls, causing you to surge and roll through space, like an ocean wave traveling toward shore. You enter a funnel-shaped appendage we call the outer ear, where you are amplified and tossed onto the tympanic membrane - the eardrum - causing it to bibrate.
No longer a sound wave, you have been transformed into mechanical energy, setting in motion the three tiniest bones in the body, the bones of the middle ear: the hammer, the anvil, and the stirrup. You collide into the hammer, forcing it into a cuplike socket on the anvil, causing the stirrup to act like a piston pressing against the fluid-filled, spiraling labyrinth that is the inner ear.
As you swirl through liquid, you reach the inner ear and snake your way through a snail-shaped tube - the cochlea - which contains fine hairlike projections that trigger nerve cells. Now you are electrical energy speeding to the brain through the auditory nerve. Countless neutrons are triggered, associations mde, and ... amazingly ... someone hears.
From: Discovering Spirit in Sound