If you're on OS X or have access to a Mac, you can make the operating system voices do waaaaaaay more than you'd think using the Repeat After Me dev tool. They won't sound like real voices, obviously, just computer TTS voices doing crazy stuff, so if you want realism it won't do. Also, Plogue makes something called Chipspeech that is commercial but may fit the bill. Of course there's always Vocaloid, but my experience with it wasn't great. I've heard impressive results though.