I think with your days question, it could be a high possibility that the some of the text scenes in the game got upgraded to cut scenes which means they need the voices, I also heard that Jessie thought that he was recording for a new kh game, and not a remake which makes me think he had some new lines

and was Roxas in any of the final mix scenes he could also be recording lines for that aswell, although ive never played final mix so cant be sure whether he was in some of the new scenes in final mix. Oh wait... it's not kh2 just answered my own question.
the more that is revealed on this the more interested I think it's the draw of final mix.
Edit: I see Sora on that throne i just think game of thrones.