Sound is the movement of air particles created by a vibrating source.
Air particles are in constant random motion, exerting very small pressure variations around the steady-state atmospheric pressure.
Each particle is subject to both an inertial force (due to its mass and acceleration) and a force which tends to restore the particle to its resting position (due to the elasticity of the medium).
When an object - a sound source - is set into vibration, each air particle moves to and fro about its average position along an axis parallel to the direction in which the wave propagates.
Air particles themselves do no move very far, they simply transfer pressure changes by what is referred to as sound propagation.
This constitutes what we call a 'sound wave' which moves away from the sound source at a velocity determined by the medium.
The velocity of propagation of a sound wave in air is about 344 meters per second, while in water it is 1437 m/s.