Why can't Finder quicklook the mp4 I just encoded with ffmpeg?
You may not have the same issue, but in my case, the problem was that the
handler_name cannot be more than 29 characters. To fix, you could try a
command like this instead:
ffmpeg -i 'My Movie.mov' -metadata:s handler_name=SoundHandler vid.mp4

https://trac.ffmpeg.org/ticket/3623