In general waves can be described as a disturbance that travels through a medium. For sound to be described as a mechanical wave it must travel through a medium, such as air or a liquid. Secondly it must have a source. For example when you hear a car horn the source of the noise is the horn in someone's car. And finally there must be particle to particle interaction. Since sound travels through a medium it causes the particles to move around and interact with one another.