I don't know how widely available this is, but amazon.com had a 30-day trial of audible.com, with the inclusion of 2 free audio books. I used it to listen to New Blood and Hunters in the Dark.
For anyone on the fence about buying them, it might be worth checking amazon.com to see if you qualify for the Audible trial. Just don't forget to cancel the subscription before the 30 days are up!
http://www.amazon.com/Hunters-Dark-HALO-Peter-David/dp/1476795851