I had a problem with mplayer today where it would not play any video, but I could hear the audio from the wmv files for my mgq302 class lectures.

After reading a little bit I found that I needed to enable a particular codec at compile time, specifically the ones for the type of video. I found this site that does a great job of explaining all of this: http://gentoo-wiki.com/HOWTO_Emerge_MPlayer