@AnswerMan, I once tried out for a series of books on tape myself. In my case, I had to work with some other actors on a scene from a murder mystery. The producer called each one of us aside and told us something about our character that the others weren't supposed to know. I was asked to be a little slow in the head, for example. The female character was supposed to be the real killer, but she had to pretend to be innocent. We all played the scene in a tiny little room with microphones everywhere.
A friend of mine actually got the part I auditioned for, but he had some training at a voice talent school and they liked how he sounded over the microphone. I did get some work doing local radio and cable TV commercial spots, however.