I'm thinking sinusitis here. The sinuses go all over the place behind you face - you'd be surprised! When you talk, the air in the sinuses is what transmits the sound of your voice to your ears through the temporal bone. In sinusitis, the sinus canal gets blocked up with, to use a technical term, goop. This is sticky mucus like stuff which the body produces as a result of the infection. This liquid makes it sound like you're underwater.
Ultimately it will go away but if you've had this for a week or so, get back down the doctors, tell him things have not improved and gently suggest that maybe a course of antibiotics might help things. Also drop that you think it could be sinusitis - that should be enough to get the Doctor to re-evaluate his diagnosis of a cold.