What are you trying to sync? Music and animation, voice and lip sync, something else?

Sync only works if the animation and the sound are on the same timeline, and not much else going on in the movie at the same time. The sound has to be on a layer, in frames, stretched out so you see the blue jagged line. Start the first frame where you want the sound to start and set the sound Sync to Stream. Any timeline over 1000 frames will start to drift out of sync. Also, slower framerates work much better. Like 12-15 fps.

More here.

If you are trying to lip sync, the only way to get it good is to use an aftermarket program like FlashAmp. This outputs a txt file that you can use as an #include or copy the code to the actions panel. It forces the animation to match the sound. You can even use an mp3 with loadSound to keep the file small. Also, with some sound programs, you can add markers to the sound file which can be read and used to force sync back on track or to raise or lower the volume as it moves along.