I always assumed that it was the jawbone conducting harmonics of a sort, and helped you to keep relative pitch. Relative to others that is. I knew a man who reckoned to tell the quality of a watch by clenching it in his teeth and sticking his fingers in both ears. If you can do this while making sure that your lips don't touch the watch, it's amazing what you can hear. Don't try this with a digital watch, only works with good old fashioned mechanical movements.
Tick Tock....Giok