What makes sound?

Sound is produced when the air is disturbed in some way, for example
by a vibrating object. A speaker cone from a hi-fi system serves as a
good illustration. It may be possible to see the movement of a bass
speaker cone, providing it is producing very low frequency sound. As
the cone moves forward the air immediately in front is compressed
causing a slight increase in air pressure, it then moves back past its
rest position and causes a reduction in the air pressure (rarefaction).
The process continues so that a wave of alternating high and low
pressure is radiated away from the speaker cone at the speed of sound.