W1zardK1ng you're my hero.
I've had Voice for a while but struggled to find Audiobooks for it. I had no idea you could download from Audible.
Up until now I had Audible in another profile. It was very annoying when I would get something like a Signal message in my main profile and would need to stop Audible to take a look.
I found an alternative conversion process for anyone that doesn't want to download and run that project on their machine. This is for Linux with ffprobe and ffmpeg
$ ffprobe <your audio book>.aax 2>&1 | grep checksum
will show checksum == <the books checksum>
Then you can paste that checksum into this page https://audible-tools.kamsker.at/ to resolve the checksum.
It will find the activation code for you and show you how to run the conversion on your operating system.
Mine were of the form
$ ffmpeg -y -activation_bytes <activation value> -i '<aax file>' -codec copy '<output file>.m4b'
If you use m4b instead of m4a you should also get chapters showing up in Voice.