Freeware to change audible.com purchased book to mp3 for the clip

Thank you for the answers but I failed to mention I have a mac so my audible.com files are downloaded into an itune format of a protected aac file which is unreadable with my Sansa.  I can burn an audio CD and rip it back into itunes without the drm to use it on the Clip but this requires 5+ cd per book.