I just experienced the same problem. Ram preview was working fine all day and then suddenly A/V went out of sync. After tweaking every preference setting imaginable, I tried closing / re-opening the project, still off… I tried relaunching AAE, still off. Finally I just restarted my computer and that fixed the problem.
Funny how a restart was the last thing I thought to try–I don’t think I’ve had to restart my computer to fix a glitch ever since I switched to a Mac 5 years ago! Back in day when I was using a PC (a time I refer to as the pre-enlightenment, stone-age years of my life), restarting was common practice. Now it’s just annoying.