The Audible listing under Services & Plugins does not work in MC18 (It says download the latest version of MC--which is wrong).
Meanwhile, one of the main reasons for using MC is your outstanding database. Of course I can use audible manage; YOU (i.e. MC) use audible manager; but that defeats the database capabilities of MC. Moreover, even now, if one downloads audible aa files (which are lower bitrate than aax files), you read all the relevant information from the file, including the thumbnail image & course information. I can't see any reason that if you've done it for aa files, you can't extend it to aax files. This should be close to a no-brainer. After all, this is a database that doesn't care about the bit rate; that is handled by audible manager.
At some time in the past, MC recognized aax files, but read none of the auxiliary information that you now read from aa files.