VLC and GOM Player use built-in codecs. WMP11 and MC12 use the installed DirectShow filters and if WMP11 can play the files correctly then MC12 should be able to do the same.
Is the file extension ".divx"? If yes, you could try to change the extension to ".avi".
AFAIK, MC should use Direct Show with both file extensions by default, but you can check that in Options > Playback > Direct Show Playback Settings...
You could test the files with the
GSpot codec information tool. If GSpot can't render the files properly you could try the
CCCP codec pack.
BTW, are you trying to decode the AC3 audio stream on the PC or are you trying to send it to an external decoder through SPDIF?