I'm not sure as to how technical you are or what your abilities are but you should know that any audio signal can be traced, it's a matter of using the right test equipment to locate it. It's not anything that you have tried replacing so try replacing whatever you haven't replaced. Each piece can be tested individually so make sure you do that first.
If everything checks out fine, the problem is inside and should be taken to a technician who has an oscilloscope as he can visually trace the audio signal to find the problem. If the problem is only with the DVD player and not the rest of the sound, it would likely be located somewhere between the input on the back or your monitor to the audio preamp or audio IC on the board. This is a guess but it's a good one