Cool! I'm working with them too! I had that last test rejected yesterday so I've downloaded Audacity, as they suggested, and have finally got the levels between -18 and -23.
This is my 30th submission so I too was really mystified by the rejections... The lady from AR was really helpful, said that Audible were coming down and not accepting anything outside their requirements - maybe they're being inundated with audiobooks and need to clamp down.. who knows .. Anyway great chatting and all the best with your audiobook business!