Flash Audio Out of Sync

I searched for a solution to this problem for a long time and finally found this...

Hi everyone --

Here's the solution to your problem with a brief explanation as to why it happens.

The H.264 is not a completely "contained" compression method -- by this I mean that Audio and Video tracks are still treated as separate entities that only QuickTime can recognize integrated timing information to match each track with one another. Flash, however, can not detect this information and so when it sees a gap of no audio, it shifts the remaining audio to fill in the gap, thus causing a sync issue. Why? Not entirely sure -- it's a Flash issue that has never been addressed by Macromedia/Adobe.

HERE'S HOW TO FIX:

Create a white noise track (generated in a sound program like Soundtrack) that is the complete duration of your video tracks and insert it underneath your currently existing audio track. Set the level to around -70dB thus making it inaudible. This track acts as a binding track to keep things in sync. Export as a self-contained file and compress as you normally would. You can still use H.264 as your compression method.

If you've already done the editing there are methods to automate inserting an audio track with already rendered/exported H.264 files. You will need FFMPEG, SOX, and a programming language (our department used Perl) If you have these programs then you can write up a short script that will input the source file, output the audio, create a whitenoise track, mux the whitenoise to the audio track, and re-insert the new audio file to the source video.

Source: http://www.longtailvideo.com/support/forum/Bug-Reports/14803/h264-audio-and-video-out-of-sync