The voices during the voice-over parts (when Tidus re-tells the story) is still done by James Arnold Taylor. It is just him trying to act serious. You always sound different when you talk to yourself. Unlike when he talks normally in the game, when he speaks in italics, he is focusing on which words to say, and how to say it. It's not unlikely that he is actually speaking to Yuna before the Dome, which could explain a little bit.