I'm having the exact same problem, but I can get more specific.
It happens whens I merge external Lav audio with my first video, then I multiclip the newly made Merged clip with the second video using the "IN POINT". The Multiclip audio plays in sync when previewing, but as soon as I make a "mark in" point, audio goes out of sync. But, if I drag that 3-point edit to the timeline, it goes back in sync... which is odd.
My theory is that since I'm using "In Points" to create the multiclip, as soon as I set a new "in point" it wants to re-sync it to that in point... which is not good.
Also, I noticed it only happens when I've merged a camera angle with my Lav Mic Audio (like how you make multiclips in FCP7). Audio doesn't go out of sync if you just Multiclip 2 cameras and no external audio source.
PS. I've done the step by step Multiclip creation from the tutorial videos and I get the same thing happening.
Still no solution anywhere in the forums or when searching google. Any help would be awesome.