Do you mean that the sound's out of sync or that there's no sound? If there's no sound, you might need codecs for your media player. Which player are you using? Have a look at the thread below for links to heaps of codec packs (some people like the XP codec pack for WMP & the K-lite pack's popular too).
CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers

That thread also has a link to VLC player. It's free, has codecs installed & is very reliable

And here's a link for Vista codecs, just in case you're using Vista.
Vista Codec Package v4.4.8 (freeware)