Sean,
I've attached an image from the DVDR owner's manual from the troubleshooting section (page 42) that deals with low sound. What stood out to me was the mention of having Virtual Surround turned on or off when listening to MTS broadcast programs. You might want to check that out.
Also, in case you don't have (or can't find) your owner's manual, you can download it from Panasonic's site
here.
Adam
