Voice activation is the default of all popular VOIP products. You have to specifically join a voice channel, and it will display that on the bottom left if you're in one. Mumble, TS, Skype especially, all of those use voice activation in setup first. Its the norm, and people expect it, and moreso, want it. I hate voice activation, personally, but with how Discord is used as a group chat in replacement of Skype, it makes sense to default it.