OK, those of you wanting to convert Audible .aa files to any other format (.wav, .mp3, etc.) need to download River Past Audio Converter version 5.0.0 from here: http://winsite.planetmirror.com/win95/audio/audioconverter_setup.exe This is the direct download and it works!!! I just used it, and coverted a book call sales and marketing. It wouldn't let me convert until I paid $29.95 but trust me, it's worth it.
http://forums.afterdawn.com/thread_view.cfm/103313 the above thread has discussed at length the topic you are bringing up !