How can I prevent YouTube from distorting the audio in my QuickTime video?
The problem was that different clips had different frame rates. My phone records a variable frame rate around 30 fps, the Zoom video had a fixed frame rate of 25 fps, the FB live video was variable around 30 fps and the Acapella-via-FB video was fixed 30 fps. I fixed the problem by reencoding all clips to a fixed frame rate of 30 fps with ffmpeg -r 30. (I also set a minimum audio bitrate of 96 kb/s, the bitrate of most of the original footage, with -b:v 96k. ffmpeg defaulted to an audio bitrate of 69 kb/s.)
The QuickTime player refused to concatenate the frame-rate-adjusted clips, so I concatenated the clips in batches of 32 with ffmpeg's concat filter and then concatenated the result of that with ffmpeg's more efficient concat demuxer. (Concatenating in one pass with ffmpeg's concat demuxer dropped the audio entirely from some scenes; I didn't take the time to figure out why.) I was able to combine frame rate adjustment and batch concatenation into one ffmpeg command, i.e. one reencoding step.