The DVD player itself will have a 'DVD PLAYER SETUP' menu which allows you to set your TV type (4:3, 16:9 Letterbox, 16:9 Pan & Scan), your audio options (surround, stereo, etc.) as well as some other options.
This is where you want to set your audio up to let the player know that you are locked into a stereo only setup for audio.
It should be easy enough to do, and if not, then spend $50 and get a new DVD player from Sony or some manufacturer that knows what they are doing.