Some video files I downloaded have no sound. Dolby AC3?

I recently downloaded some videos and am having some trouble with the sound encoding. The video itself works fine, but there is no Audio. It pops up saying I may need a Dolby AC3 decoder or something like that. I am trying to use DivX plus and or WMP. Neither has sound. There is a sound track present in the file, but I cant figure out how to get it to play with the video.