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?