Audible .aa files are transferred directly via the Audible Manager…but in the case of the Mac, I see your difficulty: the files are DRM encoded, requiring MTP mode, and that means Windows Media Player.
I would contact Audible tech support, as there may be a software option for the proprietary file format, and transfer via Mac.
I know that once the files are written to CD, they are handled as standard, unbridled MP3 files. The disadvantage is that they will lose the built-in functions like speed and chapter mode…you can squish them down to an acceptable size, by writing the MP3 files at 64Kbps.
With the upcoming firmware, the podcast / audiobook folder will help with books in MP3 format.
Bob :smileyvery-happy:
