There are rumors that the narrator is Ben Cooke, it is said he was suppose to play Avatar Roku in the movie until Shyamalan canceled Roku's part, instead inserting the spirit dragon voiced by John Noble.
There is no hard evidence backing it up though, because Ben was not credited for being the narrator. He is known to help out with stunts on the movie though. So no one can really be sure to be honest, but that is what I have heard.
Ben Cooke? Or not? I can't be sure, but that is the rumor. Hard to say for sure when no narrator was credited in the movie. *shrugs*