To get this you must visit Tifa's house during the Kalm flashback on Disc one and interact with the piano, saying you jammed a little.
On disc 2, after Cloud rejoins the party, return to Nibelheim with Tifa in your party and play the following:
(
+
) (
+
)
(
+
)
, then options to end.
Tifa will mention she has the sheet music, and the item is found within.
FYI, this did take multiple attempts before it finally worked. If it doesn't work the first time, keep trying!