As I mentioned in my How to find multiple filenames with Linux find tutorial, you can use
find command syntax like this to find files with multiple filename extensions:
find iTunes \( -name "*.mp3" -o -name "*.m4a" \)
As that command shows, I ran this
find command to find all of my music files under my iTunes directory, including .mp3 and .m4a filename extensions.
While I’m in the neighborhood, this is the full
find command I use to backup all of my iTunes files that have changed or been added in the last 180 days:
find iTunes \( -name "*.mp3" -o -name "*.m4a" \) -type f -mtime -180 -print0 | xargs -0 tar rvf NewMusic.tar
There’s probably an easier way to do this, but that backup command works for me.