Sound is a type of energy made by vibrations of tha air molecules; this means that, when an object vibrates it causes its surrounding molecules to move and in turn this molecules move their surrounding molecules making them vibrate as well. This reactions is what we called sound waves and it keeps going until the molecules run out or energy. Now, if your ear is within range of such vibrations you may be able to hear them, this depends on the speed of the vibrations; the slowest vibration a human can hear is 20 vibrations per second and this would be a very low pitch sound; the fastes vibration a human can hear is 20,000 vibrations per second and this would be a very hight pitch sound.