I'm GMing a game, and as hard as I've tried to not have it happen, sometimes several NPCs end up talking to each other or to the PCs at the same time. I'm not very good at making funny voices or changing my voice, and adding "x said" to everything takes us out of first person. So, I've looked around for some live voice-changing software. Unfortunately, all the stuff on Cnet looks incredibly sketchy (astroturfing, false advertising).
Is there a particular piece of software you guys would recommend, for either win7 or mac? I'm playing on roll20/G+ hangouts, if that makes a difference.
Edit: We're not using video, so visual-based solutions are a no-go, unfortunately.