That usually happens with low quality audio - for example if you've imported an mp3. Flash automatically compresses the audio to 16kbps, which is pretty nasty quality.
Try raising the quality of the audio like this:
- Go to: File > Publish Settings
- Click on the Flash tab
- In the Audio part, you'll see two "Set.." buttons
These allow you to adjust the quality of the audio for
stream and
event sounds. Raise the quality and test the movie again. Hopefully the sync will be better, if not fixed completely. One little catch though: the higher the quality, the higher your file size.
ps. if you've imported an mp3, it's already compressed, so you might want to disable Flash's compression.