My understanding is that is not how this works. The author pays the voice actor, or the publisher does. Amazon is paying for hosting an audio file, basically, and the development of the player app, and that's it.
I think in the very early days, Amazon may have paid all the voice actors itself (because no one was on board with audio yet at that point), and the 40% royalty model may have made sense. But I believe that is not how it works now, and 40% royalties make a lot less sense in the world we now live in; it seems to be just purely predatory.