The problem: The uploaded mini-DV files encoded with 48 khz audio sound good. Files at 44.1 khz don't produce any sound.
Since the sample rate was the most salient difference between these files, I concluded that upconverting 44.1 to 48 would solve the problem. It didn't. Now the formerly 44.1 files have 48 khz sound, but I can't hear it, either in QT-Pro 7.6.4 or Final Cut Pro 6.06.