Consonance, which rules every modulation of music, cannot come to be without sound; sound, in turn, cannot be transmitted without a certain pulse and percussion; the pulse and percussion cannot, in turn, come to be at all unless motion precedes them. For if all were immobile, it would not be possible for one thing to push against another, such that the first be impelled by the second, but if all were to be immobile and without motion, it would necessarily follow that no sound would come to be.
Therefore, sound is defined as percussion of air which does not cease until it reaches the hearing organ.
Boethius, De Institutione Musica (quoted from here)
