Open the movie file, then right click on BSplayer and select "Load external audio file...", then choose the MP3 file for that movie and in the (right-click) Audio sub-menu list you can choose which track you want to listen to.
Anyway I suggest you to link together the movie and the audio stream by using VirtualDubMod or other software (internet is plenty of useful guides, maybe take a look at
www.doom9.org too ;) )