It simply sounds to me like his voice is being processed and amplified through the head gear. Which seems like a perfectly reasonable thing for it to do.
And if it DOES sound a bit more disembodied, all the better. Just makes him a little more scary and intimidating that way.
In the 2nd trailer Bane sounded more menacing
You could tell that the mouth piece was blocking his voice from sounding normal.
Now it's like the mouth piece doesn't even exist. There's no obstruction.