Control bluetooth volume on phone?
See if this helps. I am not sure though:


  Settings >> Sound >> Volumes


If this doesn't help try this Bluetooth Volume app. This should do it.

And you are on Android 4.2?! WHAT??! AFAIK as of now 4.1 Jelly Bean is the latest... And 4.2 was supposed to launch in tomorrow's Google event. (The event is apparently cancelled) How come you are already on 4.2?