Aqua's voice was always a point of contention when the English version was released. I found her voice really stiff and pretty bad in the beginning, but got better as the game progressed, and I really appreciated that she actually sounded her age, rather than like a 13 year old.
Roxas sounds pretty terrible in that dub movie, though. I don't remember his voice being like that in KH2. Maybe it's just the quality since he basically ripped the voices from another movie and put it onto the trailer?
Giving the actor's the benefit of the doubt though: No natural earthly dialogue sounds like KH dialogue. The amount of time's "heart" is said in some of the most saccharine cutscenes ever written for a game can't make it easy for the actors....
But I definitely think Aqua is one of those "a good voice -- but bad actress" cases. Roxas kinda dips in and out, he's got a "cool and youthful" voice, so it suits, but the acting quality is on/off.
EDIT: omg, that vid Ygg posted. Now THAT'S a fan! Haha, I can't believe he managed to cut up game audio into a trailer so quickly