It's crazy people still ask for this. Think of another sandbox game that has voice chat and don't say GMod because that's a mod on top of a game with voice chat.
Voice chat is for being able to talk to teammates while you're doing something without having to stop and type - there's no use for that in Blockland, it's a sandbox game. If you want voice chat, launch via the steam overlay or use mumble or skype or teamspeak or whatever.
Loads of people in this thread are trying to compromise by saying "yeah we need muting, only approved players can talk blah blah" well all the options I just gave do exactly that.
It's not being added, ever. I guarantee it.