I don't know. I don't really like it. I liked his gruff, no-nonsense voice from the first game.
Which is why I'm partially assuming the voice is different because it's a flashback scene and Altair is still a novice at that point. It's like how Ezio had a different tone to his voice in the flashback scenes in Brotherhood.
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http://images.cheezburger.com/completestore/2011/8/22/2094c2de-bae8-4f5b-9f27-67eaff0aaeaf.jpg