can you confirm the files that play with no sound in MC actually play with sound in another program? if not, bad files.
if so, that's strange. I play .wtv files all the time in MC, so I know it works. Perhaps it's different for non-US files for some reason, but I doubt that.
The next thing I always do whenever I see 'weird' issue like this is to restart the machine, and make sure they play correctly locally (NOT over the network).
if you still have trouble, maybe the MediaInfo for the trouble file will help identify the problem.