Audible only sponsors recordings of books that are in the public domain (not copyrighted), hence we have all GMac’s books (or nearly), but none of CS Lewis’ books. So, if Tom wanted inescapable to be recorded, that would be something he and his publisher would need to talk about and make happen. Sometimes the author reads his own book; sometimes a professional reader might be hired, or of course it’d be possible to have a competent friend/acquaintance volunteer to read it.
Same with Robin’s work. If Robin and his publisher agree, they have the rights to do that but no one who doesn’t hold the copyright on a non-public domain book is permitted to release an audio recording of it, even a free one.
I think that either Toms or Robin’s books would do very well in a sound recording. Some books don’t; for example, a book with many scripture references where the verses themselves aren’t printed out. At any rate, it’s something I’d definitely love to have, but it’s up to the authors and anyone else (publishers) who have some legal right to say what happens with the book.