ffmpeg -activation_bytes <your magic bytes> -i inputfile.aax -vn -c:a copy output.m4a
What's more, Audible won't allow you to sell your audiobooks through their store unless you allow them to wrap your books in their DRM, which cannot be removed without committing a felony under Section 1201 of the Digital Millennium Copyright Act. Every audiobook you buy from Audible is locked to Amazon's platform...forever. They can revoke access to the book (they've done this with Kindle ebooks, starting with -- I shit you not -- George Orwell's Nineteen Eighty-Four... You can't make this stuff up!).
He could live in a country without those pesky US laws or be posting from international waters.
In the UK it's only a criminal offence if it's done as part of business, or you do it so much you prejudicially affect the rights holders. This is why eg people who buy a DVD player can change it to region free mode (they're not committing a criminal offence) but the shop who sells the player to you can't tell you how to do it (they would be committing a criminal offence).
Read "F2296 Devices designed to circumvent copy-protection" and "F4296ZA Circumvention of technological measures", and contrast those with "296ZBDevices and services designed to circumvent technological measures".
https://www.legislation.gov.uk/ukpga/1988/48/part/VII/crossh...