WMP for the Mac OS has not been updated since '05. It is old and no longer supported.
Anything that requires WMP and will not play with Flip4Mac will be due to some DRM that has been implemented. In which case, there will be nothing in OS X that can play it - including the last WMP. Most of the DRM stuff, MS has been trying to get those companies limiting themselves to Windows to go the route of Silverlight, which is playable in OS X.