How to use my mic on minecraft ps3 version?

Plug it in?

This is a bug, that's present in the latest version of the game. Mojang has stated that they are currently looking into the issue. To temporarily fix the issue, (this has been said to work by other players, including myself) before the player who joins the other players server; have that player connect a second controller to their console, and attempt to join your friends server through the main menu. A screen should appear asking you to add another controller/sign in. Don't add it though (unless you indeed wanted a 3rd player), instead just press X to continue by yourself. When you join your friends server, your mics should now be working. Before trying to join though, your second controller MUST be plugged in first.

Either way, if this doesn't work; sometimes restarting the game can also fix it. Though, Mojang has said for sure, it's a problem with the game thinking it needs a third player…

Hope this helps, and good luck…=D