get GSpot (beta version):
http://www.headbands.com/gspot/
Open with it a movie that you don't hear sound in.
This program will tell you what codec was the video and audio encoded with.
Look to "audio --> codec" section.
Then go to this topic:
http://forum.bsplayer.org/viewtopic.php?t=3042
And find a decoder that is needed for that codec.
If you don't succeed, post the codec information here.
If by any chance the file is mkv file then you need to install matroska splitter.