Default [Q] D800's extra mic?

Hey. I've got a D800, the AT&T variant. The only difference between the D800 and other GSM variants is the extra rear mic on AT&T's.

So what does it do? It has the other two mics present on D802, so it's a random third mic whose purpose eludes me. Does it record in mono for the camera?

I'm asking because I'm considering a textured red back replacement, but it lacks a hole for that mic.