The cause of the Vader suit is severe damage from a volcano related accident as Joeiss said. People have known that for years but I don't know if that's because George said something about it or someone near George leaked the detail? Still doesn't answer why his voice becomes so low and evil sounding...perhaps it's a synthetic voice or his normal voice is assisted by some gadget. It's been a while since I've seen Return of the Jedi, does Vader's voice sound different when he takes off the helmet?
