James Earl Jones' voice has definately got a 'Jedi' feel to it (his voice changed through each film of course and its fairly easy to spot). Maybe it was stitched together from outtakes, but since John Williams got to score this scene then it suggests perhaps the dialogue may have been completed too?

All conjecture until it's actually out, but I guess they got Jones in for a day or so of Jedi recording during the original dubbing sessions, and it would of course make sense to record too much, so they can chop and change it later. Perhaps he dubbed the cut scenes?