We'll need much more information. Is the content an actual video that you have imported into Flash? Or, is the content visuals that are animated on the timeline and the audio is either voice or music that is placed in the timeline also? Or is it something else?
The video is an animation that I created with flash. The audio is two differente songs.
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Flash composites the visual portion at runtime and decompresses the audio for playback when it loads, unless the audio is streaming. In your case, the audio is not streaming. This method of playback is more processor intensive than playing a digital video file. This means that your computer, or your user's computer has to do more work to play the visuals than it does to play the audio.
If the visuals can't keep up with the audio, then you need to find a way to reduce the amount of work that needs to be done. This might be to reduce the frame rate of the movie, or reduce the complexity of the animation, or to change the pace of the animation by showing high rate animation in short bursts and then show some low rate animation. Give the processor a chance to catch up.
Animating blocks of solid color is simpler than animating photos, animating small objects is simpler than animating large objects, animating 5 objects is simpler than animating 50 objects.
These days it's fine to stream a video. You may want to export your animation as a movie and combine the video and audio, then just display that video of your animation. The file size will get larger but the sync will be fine.
You can never depend on a users machine to play back realtime rendered animation at exactly the perfect speed so syncing music to an animation perfectly in all system and browser combos is nearly impossible.