The Internets talks about an AAX+ format, too. DRM notwithstanding, are they compatible with any standard or 3rd party player?
Audible maintains a list, on their web site, of payers that will work with their downloads.
There is a link to their list at the bottom of the page you mentioned.
wikipedia says .aa is usually mp3 or a speech codec.
The older .aa formats are apparently either ACELP or MP3, wrapped in Audible's proprietary DRM.
People say the 64kbps Audible Enhanced format is AAC-based; I can't find any reliable source for this but it obviously makes sense given that the format was introduced in 2009, is in a bitrate range where AAC has few competitors, and received device playback support fairly quickly.
Though this hole has obviously been fixed since then.
The main point: the DRM is added when the file is downloaded, on the client side.
So I toyed around downloading a few files then aborting the downloads in the middle, this is what I have found:
Enhanced AAX files: VLC player neatly showed the chapters, so they are highly likely M4B, aka. AAC book files. Just like files from Downpour/Blackstone. They started to play though no sounds was heard. I don't know much about AAC files, this is the way they are supposed to work?
Lower levels: Could not play them at all. Strange.