audio in Quicktime not playing

For some reason, lately, when i playback AVI files in Quicktime, the audio doesn't play at all, only the video. When i play MP4s and/or MPGs, they work perfectly. What could be wrong with the AVIs?

try downloading Perian, it extends the codec support in quicktime

perian.org/

also you might as well download Flip4Mac while you are at it, adds support for WMV files

http://www.flip4mac.com/wmv_download.htm