The simple answers is:
Even though Microsoft originally marketed WMC as a "one size fits all" media solution,
in a real world, user environment, it is NOT and NEVER has been.
WMC will flawlessly play many media formats, but it is far from perfect and may or may
not play certain files even if you have the proper codecs installed. That's just the way it is.
And it gets even less flexible when it comes to trying to play media files on a V1 extender
(MCX)
So my suggestion to you is to just "get over it", because there's nothing you or anyone here
on TGB can do to change things.
What I have found is that the latest version of DivX (version 7) will play pretty much
anything you throw at it, with the exception of H.264 files which to me is kind of odd
as it's represented as being able to play them.
What happens on my system is I first get an error message stating that I need "direct show"
for audio processing and then they play without audio. Now once again, the odd part of it
is: before I installed the latest DivX codec, I could not get H.264's to play at all & now they
play fine in WMP , but NOT in the standalone DivX player.
The point I'm making is this: we live in an amazing time of technological advances but not
everything works as it should. So, my approach is rather than banging my head against a
wall, trying to force something to work, I figure out which players/decoders/codecs work
(& look) best with each of the media types I wish to play & then I designate that preference
as the "default" by using the "open with" command from the right click, context menu & call it
a day!
Just my .02, YMMV