Hey All,
I have a very similar problem when I exported to blu-ray. I've been working on a wedding with multicam clips (23.98i, stereo 48k), and during one of the speeches I dragged the multicam audio all the way down and instead added speech audio file (wave, stereo, 44.1) on the the timeline and synced it manually. The issue comes when export I it to blu-ray (1920x1080, 23.98fps; stereo, 48k) the audio is distorted and crackling only during the parts where the audio files is placed on a timeline where there is a multi-cam clip. Strangely this only happens for speeches, music soundtrack (mp3,44.1k) don't create the crackling distortion.
If I open angle editor for the multicam clip and add the same audio file into the multicam clip and export, the audio is fine. But if I place that same file under the multi-cam clip on the timeline and export it to blu-ray, the cracking and distortion comes back. Also note, the audio sounds fine during editing in fcpx (10.2.1), it's not clipping, as well, when I export it to DVD or H.264 file, these problems don't occur. Only during FCPX blu-ray burning does this occur.
I think this is a bug. Hope Apple has a solution for this.
Specs:
OS X 10.10.3
FCPX 10.2.1
External Blu Ray Burner - Samsung SE-506CB/RSBDM External USB 2.0 Blu-ray Write
Project - 1920x1080, 23.98p; stereo 44.1k