Sound begins when an object vibrates and sets into motion molecules in the air closest to it. These molecules pass on their energy to adjacent molecules, starting a reaction - a sound wave - which is much like the waves that result when a stone is dropped into a pool.

The transfer of momentum from one displaced molecule to the next propagates the original vibrations longitudinally from the vibrating object to the hearer.

longitudinal: running or placed lengthwise: opposed to transverse: running side to side

Sound travels in a medium by a process known as wave propagation.