Hi.
It's a frequently reported problem.
You may want to check out our software, Flix, which will encode video into MX swf's with perfect sync.
Flix will also create FLV files that can be imported into Flash. The problem with those is that when you import it and then export from Flash, it goes out of sync again.
But you can use the Flix MX video output with Load Movie and then retain your sync.
jb
http://www.wildform.com/flix