Mumble only doesn't work correctly when you don't set it up right. It's very precise and takes a good 30min to set up properly.
However, I'd say no to teamspeak/mumble/vent. I don't even want to imagine how much abuse those channels would get without someone constantly being there to monitor.
Most people have Skype. It's the perfect solution to this. You can easily talk via audio to people who want to talk to you on there.