Does the audio sound like it should for the movie? If so, it could possibly be a codec problem. Check the thread below for links to codec packs & also the free VLC player...it has codecs installed & plays most things. If VLC won't play the file, chances are that it's crap
Codecs & VLC

Always check a video file before download with Bitzi
How to use Bitzi

Also preview the file after about 10% download...to check that things look ok!