I have also backed up all the audio books I bought with #Audible credits. Tried #OpenAudible, but apart from the free version not converting files (so I couldn't be sure that worked), the AAX files it downloaded all appeared to not work the #Audiblex script I found.

Ended up having to re-download them all manually in a browser (including a few retries for failed downloads), then converting them with Audiblex (which was great). Find it at https://github.com/naueramant/Audiblex