Thanks so much for posting this reply. Seems FCPX (or Quicktime) has issues rendering audio where there is no video. I was trying to export a project where I had created a compound clip consisting only of audio tracks so that I could loop a soundtrack. Some of the audio tracks were over a gap in the compound clip. It would not export, I kept getting the Quicktime Error -50 immediately after trying to export.
After reading your reply, I tried disabling the compound clip containing the soundtrack, and it worked, so I eventually had to delete the compound clip and re-build my soundtrack edits in the main timeline, which will be a pain if I want to move any of the clips around in the future. Oh well, at least I got it to export.