The rule for downloading old games for emulators is if you own the game and the system it is perfectly legal to have them. Does the same rule apply to audio books where if you have got the book (e.g paperback or hardback) can you then download for free the audio books.
No, it isn’t legal to download a free audio book of a book that is still copyrighted, even if you do own the book.
It is legal to download books that are no longer copyrighted (the copyright has expired) though.