How to convert books from Audible.com to Mp3
1. Download the Enhanced version of the audio.
2. Download AAX to MP3 - Audible AAX to MP3 Converter
3. Run the app. (AaxToMp3_CS.exe)
4. Open the file (\Audible\Downloads). If you have Windows 10 and use the Windows Store App, the file may be in C:\Users\NAME\AppData\Local\Packages\AudibleInc.AudibleforWindowsPhone_xns73kv1ymhp2\LocalState\Content
Even if you use the Windows Store App, you also need to use the old Windows app to sign in.
Then it just converts the file into chapters. Unfortunately they are not zero padded:
For a Mac go here