I too have the exact same issue.
I've had my iphone 5S now for exactly 1 month.
Every time the phone is moved more than 1 1/2 inch from right next to the corner of my mouth, the listener says I sound very distant, hard to hear and like I am in a tunnel.
I tried putting a tiny piece of clear cellophane tape over the back microphone and the problem largely resolved.
So, that Microphone is put in to help cancel out peripheral noise. So I imagine it is a software issue which results from the phone thinking my voice is pheripheral noise when I am more than 1.5 inches from the phone.
So I called My-APPLE today and they are going to set me up for a genius bar. But I demonstrated the tape-no-tape issue to the MYAPPLE guy and he agreed that the affect is horrible but fixed by the tape largely.
So now the question is, if I get a new phone will it be fixed or will this just be a tremendous waste of time.
My guess, either Apple needs a software update or a catastrophic call-back (and that won't happen).
They won't give me a refund since today is technically day 31, but with AppleCare, which I have, they will give me replacement, after replacement, after replacement -- go figure!
So I told them to tell the technicians (who I wager already know). I will come back to this thread if I hear anything.
Good luck, everyone. It is a pain, eh?