I finished this one today for the second time actually. The first time it was before I started this diary. It definitely wasn’t as hard as the first time. I finished the last level in one go, but it was still a close one. Watching the credits, I noticed a few songs in the list that weren’t in any of the levels. I’m guessing their either unlockable or in the US version of the game. If the songs are unlockable, it probably has something to do with the rank system which, right now, has no other use. It’s one more game to the list.
EDIT: After gaining the rank “Captain of Soul” I unlocked a new song. It seems it has everything to do with ranks. The problem is I need almost three million points for the next rank which is just a bit less than the total I have now. This will be a long day.