they are in various treasure chests throughout the mansion, on floors 1 and 2, as mentioned previously.

there are secret doors in the bookshelfs. to figure out where the secret rooms are, find a place on the map where there is a passageway with no door. press square by the bookcase in front of the passageway, and it will open up to let you in. there are 5 total music sheets, and you need to get behind the secret doors to get to any of them.

I think there are at least 3 on the first floor. the first is down the first secret passage you find, which is upstairs in level one, if you follow the hallways back through, on the left.

on the way to the stairs to level 2, down the 2nd secret passage area, there is a secret room on the left in a big room where the path turns right. in there is another sheet.

if you go around the 1st floor, and get the key to the locked library, I believe there is another music sheet there.

On the 2nd floor, in the NW corner, there is another secret passage behind a secret door, where there is a third.

I believe the last music sheet is in the first main room you come to on the 2nd floor.

hope this helps. sorry if my memory is off, or I couldn't be more specific.