Unfortunately, Audible audio files cannot be converted into MP3 or any other file format.The reason for this is because we utilize security technologies to protect the intellectual property rights of our content providers and authors. Additionally, they help us to provide a superior listening experience that you won't find anywhere else!
Audible is able to offer an award-winning service that is DRM-free for publishers who prefer this method and are committed to working with Audible to maintain this one of a kind customer experience. In order to best support this commitment, DRM-free titles are supported within the Audible platform using Audible file formats - either .aa or .aax."
The B2 can play mp3, flac, wav and aac.
It seems that Audible is choosing a proprietary format, probably to prevent copying!
Steve