If you are having problems playing some movies then you might need more Codecs.

Goto http://www.mplayerhq.hu/design7/dload.html
and download the Codecs for your Linux installation.

Example: Linux X86

tar -jxf essential-#####.tar.bz2

cd essential-#####

ls -l /usr/lib/win32 To check if it is there.

If not mkdir /usr/lib/win32

mv * /usr/lib/win32

That should do it.

This is where I got the information:
http://www.slackersbible.org/?q=node/92