Please supply full model numbers and if this was a problem since day one. Given the possible age I'd get repair estimates. My bet is some board has begun to fail.
I have a 61" DLP. When I first turn on my TV its volume level is very low at the setting where I last was listening (ex. 20) so I have to turn it up to twice the usual level (ex. 40) to achieve the same noise output. After about 10 minutes of being on the volume returns to its normal level and I can reduce the volume level back to its original setting (ex. 20). If I turn the TV on through my surround sound (so the sound is only coming from my surround system speakers and not the TV), there is no problem with the volume. In both cases, I am routing the signal through my DirecTV box. Any thoughts?