Sound in air propagates in form of:

Sound is transmitted from a source to the surrounding air particles, which vibrate or collide and pass the sound energy along to our ears. Without any particles to vibrate, we wouldn't hear the sound. 
Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving. If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. The motion of the particles is parallel to the direction of the energy transport. This is what characterizes sound waves in air as longitudinal waves.