That happens to my KH1 disk. It's not a glitch, a glitch being something the game does as a set of actions done by the player has not been programmed to be able to do have been done and now the game acts unpredictible.
It's only the disk being too scratched up for the laser len inside the PS2 to properly read and anlyze. Try cleaning it with soap and water, and if that doesn't work, then try to find something that'll repair the scratches on the disk. If that doesn't work, then damage has been done to the very data of the game, you see the printed side of the game consists of all the data under it, then when light reflected through the blank disk under it, it'll read.
But if that has been damaged, then there can be nothing done to fix it, as that data has been destroyed.
I hope that would help you.
EDIT: Wait... you're talking about the very end of the game... when you have defeat Xemnas and after the credits roll? Then that's nothing! No problem! However, if that ever happens that you are approaching a cutscene and you hear the music, but it never goes to that cutscene... THEN follow my advice. ^-^
