Interesting, I've never played with Twitch Voice/Unvoice options.
Twitch chat is based on IRC. In IRC, when someone has Voice, and an operator sets +m (moderated) on the channel, then only people who have voice can talk, and everyone else cannot. But as far as I can tell, Twitch doesn't have a way to set a channel as +m (maybe it can in the IRC interface? I dunno) so I'm not sure what purpose the option serves.
I wonder if anyone else has more insight into this.