well, the programmers can always program the music to pause when the game loads and resumes when its finished.s1smackdownUnfortunately a notable loading time in the middle of the cutscenes even at max processing speed means syncing Fate of the Unknown to that scene like the secret movie is impossible.
As for the Keyblade war, we do get the backstory of that in Xehanort's reports and a few of his speeches. Keyblade users of light and dark clashed in attempt to create the X Blade. That's about it. While we might get a flashback or two nothing near enough to make a whole game about.