The music suddenly changes to a semi-acapella version of the original soundtrack and you can hear singing that is a bit unnatural.
The voice is Hatsune Miku's 
You are basically saving Miku-chan from dragons and she thanks you for that. During that area, all the music is replaced by the Miku versions of the song and instead of the abrupt transitons of music with the restarting of a melody etc, Miku instead switches from song to song almost as if an actual musician would in that situation and the crazy thing is that you can hear it, as if they programmed all the loops and scenarions for that. Guess now I know why the games asks for 900+ megabytes of install data beforehand.
After you rescue Miku-chan though, you have the option to unlock DIVA mode. Basically, you construct a concerto room for Miku and once there, she thanks you and tells you that she will support you with all her love. What this does is that it unlocks DIVA mode on the options menu : DIVA mode replaces each and every single song in the entire game, all 60+ songs with Miku versions of the song, where she sings them. That's not all. If you play the game with her songs, the monsters somehow tend to miss a bit more because the in-game and story explanation is that "the monsters hearts seem to have calmed from the melody".
HOLY SHIT! You basically can quell the Dragon menace by having Miku sing to help you.
Oh yeah, you also get a new class in this game called "Idol" that basically uses cheers and songs as buffs, all-out attacks etc and some of the songs are references to some of the Vocaloid original songs AFAIK. I haven't meddled much with Idol since my team of Samurai, Trickster and Psychic are doing fine and I want some more levels before I start leveling my hackers and my destroyers.
Basically Samurai = Warrior / Fighter, Destroyer = Monk / Martial Artist, Trickster = Assassin / Ranger / Strider but with guns and gun-fu, Hacker = Buufer&Debufer class, Psychic = Mage with elemental and healing spells.