Video works in omxplayer but not xbmc

I have the most recent release of Raspbian Wheezy running on my RPi. I installed XBMC following these instructions.
The only other thing I have installed is Midnight Commander so it's a pretty fresh install.

The problem I am having is that XBMC will not play video files that omxplayer can. The video files in question were able to play perfectly fine in XBMC before I did the fresh install.

What is causing this and more importantly, how can I fix this problem?

I would appreciate your help on this.