I have a XVID video file (The Godfather) and other people havent had any problems with this file but i cant hear any speech when there is background music playing or sometimes not at all. Normally wmp doesnt play any video clip from my hdd but now even VLC isnt playing this movie. I cant remember installing any codecs other than XVID ones that come with DIVX player and other software.
That could be the way it was encoded, maybe the audio is slightly out of sync or perhaps the volume/equalizer settings were a tad strange. Or your audio codecs could be to blame... Try using K-Lite Codec Pack: http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm Install it with the recommended settings, and use Media Player Classic that comes installed with it. It's codecs are high quality and will allow you to play 98% of video formats out there. Let us know if this fixes it.