Codecs e media players

[Marcelo Magno T. Sales]

People,

I have some video files here that can be played with mplayer, but not with 
xine nor kaffeine. Why is that? Shouldn't any media player be able to play 
any video file if the right codec is installed?
I have the w32codec package from atrpms installed (it places the codecs 
in /usr/lib/win32) and i have softlinks pointing /usr/local/lib/codecs 
and /usr/lib/codecs to /usr/lib/win32.

Thanks,

Marcelo