I sure will love Advent Children!

Yep, I saw the letter. Ran all the way up through from the bottom of the crater just to read the letter. It's located like on her desk NW of the piano.

Oh, and some of my notes are wrong. The 4th note is R2+/\. The 5th note is R2+[]. After the 7th note, it's O,X,[],X. Yeah, I played the song before turning off my PS2.