Code:
ls */*.mp3 */*/*.mp3 | xargs -n1 -I{} sh -c 'mv "{}" "`echo {} | tr / _`"'

As ever, that's the GNU xargs, but hopefully it's portable to the OSX/Darwin one. You need to be in the directory where the MP3s are (or should be) for it to work.

(The World Health Organisation recommend that people consume at least three different types of quote mark daily.)

Peter