Sound is nothing more than your perception of pressure waves in the air. Your sensory system has evolved transduction mechanisms capable of converting simple pressure waves in the environment into a useful data collector, tool for environmental assessment, and subjective sensory experience.
The method of signal transduction lies in the motion of microscopic hair cells in your inner ear. These specialized cilia-like structures will bend in response to pressure waves traveling through the air. Through the process of mechanotransduction, the bending hair cell will produce a depolarization that is ultimately experienced as sound in the brain.
“Hair cells respond to deflections of their hair bundles by opening and closing transduction channels,” says Peter Gilespie in his 2009 “Cell” publication. This means that pressure waves, in the form of sound, are capable of creating an electrical current through their action on hair cell bundles. The electrical current is primarily composed of calcium ions flowing across the cell membrane.
As calcium enters the hair cell, synaptic vesicles carrying neurotransmitters begin to fuse with the cell membrane, releasing their chemically active contents into the synapse between nerve cells. This leads to transmission of the electrical signal into a new cell, which will depolarize and subsequently pass the message into the central nervous system for further processing.