One of my kid's schools had a wild debate about this for their yearly charity "Readathon." The kids get sponsors to sign up to pledge the amount of money they will donate for each book the kid reads during the readathon. Parents are expected to sign off that their kid really did the reading for all the books they put on their sheet. Other than that its on the honor system. They've been doing it for 60+ years.
The kids don't get any prizes but they do get certificates of recognition for various levels of achievement. It came up this year, that some parents count audiobooks and others don't. They have been doing it like this for ome years now but it was never discussed before. The rules don't say anything about audiobooks but they were written back when audiobooks weren't common.
We had some seriously intense discussions in the Parents Charter (the name for our PTA) and lots of people have strong feelings about this! In the end the agreement was that transparency is best so the form has been updated with one side for audiobooks and one side for books. For purpose of donation they are counted the same.