Andrew,
My audio (in 5.1) also sounds like deep in a bathroom, epsecialy when a person holding a camera talks behind it. Talking in front of the camera doesn't create such "deep" effect. I wonder if it is anything to do with the video being Dolby Digital while setting on my amplifier are "surround sound". Have you had a chance to play with manual settings?
Read not to contradict and confute, nor to believe and take for granted... but to weigh and consider.