I was there for the Heinz Field shoot.
He sounded like an old, wheezing British guy. It definitely wasn't what I was expecting Bane to sound like. I've become acclimated to the voice, though, and I think it sounded great in the latest trailer.
I'm pretty sure the way he sounded on the day of shooting was never inteded to be the way he would sound in the film. With that mask over Hardy's mouth it is very easy to change during post-production (and probably even necessary).
The voice at Heinz was Darth Farmer.