As I was thinking more about audio books, I realized there are a plethora of podcasts developed for children, many of which are free. One of my, my son's, and children with whom I've worked favorites is Story Nory. The podcasts are available for free on Itunes.
I believe Story Nory podcasts originate in the UK, and the stories are mostly read by people with British accents, which makes them very lovely to listen to. When my son was younger, we would download many stories for trips and he'd listen to them quite a lot. His favorite story was Tick Tock Turkey and The Disappearing Sandwich because it deals with time travel, one of his interests.
I've used Story Nory stories for listening at school at the end of a long day with my 3-6 year olds, where the children lay on the floor in the dark, relaxing, and I've used them in a Reading Practicum I did this past summer with 4th graders. They've all loved them. I think you will took.
There are other podcasts of stories out there for children, and some of them are free. These are the ones out of which I've gotten the most mileage. Hopefully you will too.
If you know of others that have been successes for you, please leave a comment and let us know! Thanks!