Any vibrating object in a physical medium (other than vacuum) causes movement or vibration of particles in that medium. The particles oscillate to and fro in the direction of movement of the object.
This vibration propagates along the medium in all directions around the object.
The sound travels as a disturbance in the medium. This creates waves in the medium. Only the wave or the disturbance travels from the source (object vibrating) to the ear. The particles of the medium oscillate or vibrate around a mean position, where they were located earlier to sound generation.
These vibration cause changes in pressure at each point along the path of sound wave. The sound wave travels as a sequence of regions of excess pressure (compressions) and regions of lower pressures (rarefactions).
The particles in air / medium vibrate. Then the disturbance reaches the ear. Then the compressions and rarefactions reach the ear drum. The ear drum is very sensitive to the pressure changes. So it vibrates at the same frequency and intensity as the sound wave. These vibrations are converted in to signals by our nervous system.
As the particles in the medium oscillate in the direction of the propagation of sound wave, sound wave is called a longitudinal wave.
If there are no particles in the medium, then sound wave does not propagate. The compression and rarefaction regions do not exist.