Not sure if this is related to your problem, but I one had a problem with playing DVDs off my computer through a home receiver (not through the Wii, but read on). What happened is somewhere between the DVD, the computer, and the receiver, the 5.1 sound was messing up. When 5.1 sound is decoded, all voice in a movie typically goes through the center channel, and for some reason my receiver wasn’t dealing with that right and so it sounded almost like there was no voice in the movie at all. It sounds a lot like what you’re describing.
What you could try to do is go into the DVD’s audio menu (if it has one) and try it with 5.1 sound on, and then with Stereo on. What fixed my problem is making the DVD play the stereo track, causing no surround sound but at the very least I had voice again.
I know this isn’t directly related to the Wii or WiiMC, however the Wii does support Dolby ProLogic II, which in fact does handle stereo to 5.1 processing. It MIGHT help. If not, well, at the very least that’s one thing that’s not the cause :/
