Equip yourself with a large beer & read 'man mplayer'
Usual thing in an X terminal
mplayer -alang en /path/to/file
This presumes you have installed a plethora of stuff - LibThis, LibThat, SDL, flac, ffmpeg, and a host of half assed other programs. You should be thoroughly sick installing the stuff. Otherwise, when you configure mplayer, it spits you out a list of it's capabilities
MP3 support no
video support no
Audio Decoding no
anything useful no
and that's what you get. If you type 'make' on that. . .
The solution is to install the dependencies (That's hell) and then recompile mplayer.