Lump me in with Billy and SAK. Officiating is my respite from "real life" and whilst I'm between the lines, the rest of the world ceases to exist for me. But that's just me.
If I were struggling with focus, first I'd try what I normally do when my focus is wandering (just because my mind is off the world doesn't mean it's totally on the game either). I concentrate on what's happening. "Blue is bring the ball" "White is on defense" "Left foot is his pivot" "Screen coming", that sort of thing. Consciously focusing on the action, loosing the voices in my head, tends to drag me back to the action pretty quickly.
If that isn't doing it, perhaps the game is a "cake walk", I'd think about picking a couple of things to work on.
If all else fails, you can try kicking a couple of obvious calls and let the coach have a go at sharpening your focus.
Maybe some of that might help.