Several problems with this suggestion, and please note that every single one of these has been mentioned many, many times:
1. It's not possible to make this in-game, it would have to be an external program.
2. It can be easily, and heavily, abused. Quote: "I don't want to listen to prepubescent children whining" is an example. Another example are trolls, well, trolling.
3. It would heavily lag the game. EXTREMELY.
4. There's this thing called Cyber-bullying, and this definetly wouldn't help that.
5. It's not wanted by 75% of the blockland community.
In summation, it CANNOT be done. And even if it could, it is a HORRIBLE idea. Just use skype or teamspeak if you want voice chat so much.