I setup a PC in the house with Win2k, MC 12 (the latest), Matroska Splitter (downloaded from the Haali Media Splitter homepage?!), and ffdshow_beta2a_20070322_clsid. My music collection is mp4's (m4a extension). When I double click a music file, MC throws up a QuickTime out of date window. I don't even think QT is installed on that PC. It's a fairly fresh install, and I couldn't find it anywhere. Not in Add/Remove programs, and only a few worthless entries in the registry.
I tried setting the playback settings for mp4 and m4a to no effect. (I read the wiki article.) I can load an mp4 in GraphEdit, and it plays fine. Everything looks ok in my novice opinion.
What else can I do to check to make sure MC12 has what it need configured to play mp4?
P.S. Not sure if it makes a difference, but the user is signed in under a power user account on Win2k SP4+, not administrator.