If you are takling about sound in general, audibility requires vibration abobe a certian threshold of the transmitting media and the human ear only "hears" frequencies within a limited range . 

If you are talking about music, given two sound frequencies(which may or may not be audible), a tone with a frequency equal to the difference of these two will be audible(called the beat frequency). 

If you are talking about listening to the heart, it is the vibrations caused by turbulent flow that produces vibrations->sound.