This is something that I run into all the time. Every time I reinstall in my laptop, I forget to set this little thing.
There is a bug in either Gstreamer or Beryl (not sure which) that does not allow you you watch video. I know that is worded horribly but what happens is you open a video and you get audio but no video (black screen).
(1) Open a Termial (Applications -> Accessories -> Terminal)
(3) click the Video tab
(4) change the Output Plugin to X Window System (No Xv)
if you want you can click test and you should get color bars like on your tv