As I understand it, Mplayer is the back-end for most media player applications for the tablets (means that most media player applications use it to play video, instead of having each application use some different method of playing video). Try installing Mplayer and see what happens. If you can't find Mplayer, try installing Canola or some other media player. I *think* they might pull Mplayer automatically, but I'm pretty unsure (I had Mplayer installed before anything else).