I have the same problems with audio sync. If I shoot at 25fps on canon 5dm2 the audio won’t sync with the audio recorded on my Zoom H4n at all in the event browser. It will sync just fine at 24fps but when you sync the two files, the resultant compound clip goes from 1080 to 720. I’ve looked on the Apple FCPX forum and I’m not the only one with this problem.
Fotarrgrapher