It depends how the individual DVD is mastered. Some let you switch between audio tracks on the fly, some don't. I seem to recall M&C being one that requires you to go back to the main menu to switch audio tracks.
So, in the main menu, check out audio options. Select DTS and see what lights come on on the receiver and also what speakers light up.
My suspicion all along is that the digital settings on the DVD player are whack. If you want the receiver to decode the digital signal (which you do), then go to your set-up menu on the DVD player. Go to digital audio or audio or whatever sounds like a likely candidate. The settings for both Dolby Digital and DTS should be set to bitstream - not PCM. This should set the player's outputs properly.
I've checked all that, everything is indeed set to bitstream.
