(Edit: thank you, everybody, for the great answers!!!)
(Edit: thank you, everybody, for the great answers!!!)
Librivox has free, public domain audiobooks. The narrators are volunteers, so performance and recording quality varies, but there are some very good ones there.
Some podcasts provide legitimate audiobooks of copyrighted works, generally with similar quality caveats to Librivox. For example, the blog “Readings from Under the Grapevine” has a free, legally licensed recording of the Narnia series (except for The Lion, the Witch and the Wardrobe).
I never buy audiobooks with DRM, even if the DRM is trivial to break. Amazon has made clear their intentions to lock media down ever further. I shouldn’t have to put up with that, so I’m not going to support them financially.
If I can’t find a copyrighted audiobook at one of the DRM‐free shops, I get it from my library, either through Libby or by checking out a physical copy. Not that these would be helpful for someone using Audiobookshelf…