Another one that comes in handy occasionally is afconvert to convert audio files. I like to add audio files (eg audio books, or guided meditations) to iTunes/Music or Books, and they are a little bit finicky in terms of what they accept and sync (I've had problems with files converted with ffmpeg).

I use this to convert an mp3 to a variable rate m4b for Books:

    afconvert -v -s 3 -f m4bf meditation1.mp3