Why does yawning impair hearing? When I yawn I can't hear anything happening around me. I also feel some kind of muscles inside my head contract and hear a faint hum, but it is not loud enough to explain not hearing other sounds. I can force those muscles to contract even without yawning and it produces the same effect of humming and no other sound, so I suppose they cause this effect, but how? What really happens? What muscles do I feel?

It is related to Eustachian tube. It links the back of throat and the middle ear and allows air pressure to equalize in the middle ear. When you yawn air pressure goes up in this and it bends the ear drum and causes impair hearing (notice, just impair and not stop). Yawning also helps to open Eustachian tube.