There is a great author out that is giving his audio books for free on i-Tunes. His name is Scott Sigler. Scott has 6 books out all for free.
Ancestor, Sci Fi Medical Horror; Infected, Sci Fi Medical Psychological Horror; Contagious, Sequel to Infected; Nocturnal, Crime Horror; The Rookie, Sports Sci Fi; and finally Earthcore, a Sci Fi Horror.
All of these books are good, and I have listen to all of them, and I think that any writer will enjoy them.