Years ago, I was given a Nook (Barnes & Noble's version of Kindle) with it came an optional subscription to this:
free Nook books:
https://www.barnesandnoble.com/b/fre...ok/_/N-ry0Z8qa
Not long after I received a Kindle. And signed up for free books here:
https://www.bookbub.com/ebook-deals/free-ebooks
My email inbox was filled daily (or weekly) with customized by me lists of books (about a half dozen, more or less) to download for free.
Included with the free lists were other books the algorithm picked out for me (not free). That list included the best online sites with the best prices to buy them.
I ended up with hundreds and hundreds of books-nearly all free- on those two devices. I did buy a few that were suggested.
Sorry tho....don't think they do audio....yet.
Here are some free audio book sites I found in a search, but I haven't tried any:
https://librivox.org/
https://www.digitalbook.io/audiobooks/popular
https://www.overdrive.com/ (this one I have used...it' connected to the library, and has a limited number of copies available-this sometimes requires signing up on a waiting list to check them out)
https://www.storynory.com/ this one includes lots of talking animal books
https://lightupyourbrain.com/stories/ audio stories for children
https://www.openculture.com/freeaudiobooks this one doesn't seem to have a search feature...just an alphabetical list of authors....
https://www.lifewire.com/free-audio-...online-1357952
there are others.....
Don't forget to come up on occasion for air, water & food