Why is there no sound in my avi files, and how can I fix it?

I have three avi files. Whenever I try to open them, I get the video just fine, but no audio. All other avi files I have are functioning alright. I tried opening these three files using WMP, DivX, and Quicktime, but nothing works-- I always get only the video, but no audio. Is there anything I can do to fix this? I'm not even sure what the problem is.