Asked by Ajka on the How It Works forum
When we listen to someone else speaking, it is the change of pressure in the air that our ear picks up via the eardrum, which in turn vibrates the bones in the middle ear. This causes fluid in the cochlea (inner ear) to move hair cells deep in our ears, and send an electrical signal to our brain.
Our own vocal cords vibrate our skull, and caverns such as sinuses can amplify this by passing the middle ear and going straight to the cochlea. Low frequencies (bass sounds) prefer this method of direct contact, which can subsequently make us think our voice is deeper than it really is.
David Houston, Science Museum