This happened to me with one of my flashes, and is now happening with a flash I'm working on now. It's frusterating as hell.
The last one though, someone suggested I put all the sounds in a movie clip. This ended up being what I did, though it may be a bit tedious. Just assemble all your sounds the way they are suppose to be, synced up and whatnot, then copy and paste the frames in a new movie clip...I think that's what I did, either way it shouldn't be too hard to figure out.
I too, would like to know the cause of this though.