First things first, I own nothing of this video. Not the game or the song playing. All rights go to their owners which happen to be Type-Moon.
A rather calming, and relaxing 'My Room' theme that only plays once you've reached the Lost belts. Once the Lost Belts section of the story has been reached, the default 'My Room' theme will no longer play. That is unless you unlock/play it in the sound test.