If its not the DVD player, it could be your processor's surround decoder (not likely.)
More than likely its Sony's recent reliability issues. Unfortunately their mass market electronics have plummeted quality control-wise, with perhaps most of the complaints coming from owners of their mid-low fi DVD players (I was one of them). Its unlikely that I'll ever buy another Sony A/V component again
In any event, its probably a greater hassle (timewise and financially) to try and repair it than to get a new player.
i have a sony 707 (the jap version of the 725) and had the exact same problem.
it was simply fixed in the player setting, just change the output from PCM to DOlby Digital.
If that doesnt fix it, take it in to your nearest sony dealership, if they cant help, find the nearest rubbish bin, lob it in and go buy yourself either the marantz 4100, the arcam or the pioneer 535, 636, 737 etc