I think it's not too much of a waste if I repeat here two simple tricks with #ffmpeg in order to upload audio as video to mastodon:
1) test-tv image/animation:
ffmpeg -f lavfi -i testsrc -i AUDIOFILE -shortest OUTPUT.webm
2) album cover or similar:
ffmpeg -loop 1 -i IMAGE.png -i AUDIOFILE -s 200x200 -shortest OUTPUT.webm
* with -s WWWxHHH you can adjust the size/resol. of the video
* instead of webm you can safely use mp4 as output extension - it will create an mp4 with x264/aac codecs