I don't think -f mp3 would ever work, -f is for choosing file format to download. So -f mp3 would work only if there is a source with codename mp3, which I don'

I don't think -f mp3 would ever work, -f is for choosing file format to download. So -f mp3 would work only if there is a source with codename mp3, which I don't think I've ever seen. You may be using -f bestaudio -x or something along the lines of it.