Does Stern randomly contact people? I don't really understand. I guess what they're doing is working so far, as they're able to find people who can act at least well enough to be convincing.
But what I do wonder is why they haven't ever tried to hire voice actors like Kyle Hebert, Patrick Seitz, Cristina Vee, Erica Mendez, Yuri Lowenthal, Vic Mignogna, Brittney Karbowski, and the like, who are professional, have a certain amount of star power to them among animation fans, are easy to get ahold of, and are inexpensive. Indie video game companies made on a shoestring budget and animated shorts made by two people on Flash can get these people. (Kyle Hebert in particular likes voicing for small projects and has his own recording room.)
I don't mean to belittle the people who are currently voicing stuff for Stern's pinball machines, because I have yet to find a true dud as far as voice acting goes. I simply wonder why Stern hasn't tried these people at least once.
Then again, it does result in tremendous variety of voices. After some time, it becomes easy to identify voice actors who you hear again and again, and it takes me out of the story and I start wondering if I'm correct in guessing the voices (unless they have amazing range, like Tara Strong or Matt Mercer).
(The main reason I wonder, though, is because I'm an animation fan and a trivia fan at the same time, which means I pay attention to who voices what, go to conventions, and learn behind-the-scenes stuff. These guys have lent voice work to near every medium that isn't live action, and pinball seems to be one of the holdouts.)