PC sound issues

I have a laptop that until recently was working fairly well. I’m not sure what has changed, but some files (mostly video) have no audio. However, I have another PC (a desktop) when I play the same file on the desktop PC the video and audio are perfect. I first noticed it when watching streaming video from MSN. But now I am noticing the same issues with files I downloaded quite a while ago. These files worked fine back then. And they work fine as I said, on my desktop PC. So I believe it’s unique to the laptop. Any ideas on where to begin?