Sorry for the blatant plug but when I was searching for info about converting Audible to MP3 most of the methods were long out of date as Audible put pressure on software creators to respect their DRM ('Goldwave' etc). When I eventually found out how to do it I documented it here - http://blog.davidwaterston.com/2006/08/09/audible-unlimited/

It's an old post but it still works and is much faster than any of the other methods.