I just had this problem and solved it. My gear: Canon 60D, Beachtek DXA-SLR PRO. Maybe it was a flipped switch (still haven’t diagnosed as I just literally solved the issue), but I had one XLR input. For some reason when I imported into FCP7 it created a stereo signal with each signal canceling the other out. Once I panned the audio in FCP is was fixed. Yet, if I play the files in Finder (Mac) they play fine. Maybe FCP screwed up?