According to the Encyclopedia Britanica, sound is; a mechanical disturbance from a state of equilibrium that propagates through an elastic material medium.
To expound upon this concept further; sound is a disturbance in air pressure; a vibration, energy. For example, our vocal chords vibrate and cause a disturbance, change or fluctuation in air pressure. The vibration in the form of a wave propagates from the source at a speed of 343.2 meters per second (1,126 ft/s; 1,236 km/h; 768 mph; 667 kn). Sound waves travel through the outer ear, are modulated by the middle ear, and are transmitted to the vestibulocochlear nerve in the inner ear. This nerve transmits information to the temporal lobe of the brain, where it is registered as sound.
The frequency of the sound is directly correlated to the speed of the vibration. This vibration or frequency is measured in a unit of measurement called Hertz(Hz). 1 Hertz is one cycle or vibration per second. A human being with optimal hearing can detect or hear sounds with frequencies between 20Hz to 20,000Hz (20kHz). However due to hearing damage generally caused by exposure to loud sounds which is measured in decibels; after childhood adults can generally hear up to about 15kHz.