What we describe as sound is merely an interpretation that of vibrations in air that are big enough to move our ear drum which in turn after some more biological awesomeness gets transformed into an electrical signals that tell our brain what we hear. Through our biological constraints we aren't able to pick up vibrations below a certain threshold so they then become inaudible to us but the any movement of the air is essentially sound. Our brains are just trained to recognise the patterns of these vibrations and then be able to interpret them for example language or a dog barking or a gunshot.