To be honest, I recognized Sora's voice in about five minutes (I suppose the main reason is because I'm a movie freakaziod myself) so I checked the pamphlet then and knew everyone's voice the whole game. However, I never got the chance to really hear Sephiroth's voice. But I don't know what he'd say...does he say anything after you beat him? And heck, does it matter anyway?
Personally I agree with riku104. I'm so happy with Haley Joel Osment, David Gallanger, and Hayden Panetteire for Sora, Riku and Kairi (Hayden's very attractive, P.S.) and I hope they return for the second. They all sound much older now, though; see Second-Hand Lions and Ice Princess.
I'm not so sure I'd like the second game very much if Osment didn't return as Sora. Panettiere would be understandable since she doesn't sound very much like Kairi anymore and Gallanger would be tolerable...but Osment just seems to fit Sora like a glove.
How's this for a day-tripper, though: I've never seen the actor for Ansem until a month ago I was watching pretty much the only wildly released film that he's in. Watch and listen for yourself: Ansem's voice actor is Rose's fiancee wannabe in Titanic.