You give these to Agatha, who lives in a house north-east of the Meresti Trainyard. Once she has her violin back (assuming you give it to her) these will allow her to play more songs over the radio. Other than that, it doesn’t really serve much of a purpose.
Arlington Public Library - Children’s Wing
In a room on the western side of the level, after entering from the Lobby. It’s on the floor near a stand.
National Archives - Archives Sub-Basement
In the room full of metal shelves near the stairs to the National Archives, the Sheet Music Book is on a metal shelf along the western wall.
Roosevelt Academy - Roosevelt Arts and Athletics Hall
In the auditorium on the stage, the Sheet Music Book is on the floor near a music stand.
Vault 92 - Living Quarters
In the dorm with the “Men’s Dorm Security Terminal” in front of it. Go into the second room, then north into a bathroom. The Sheet Music book is on the floor near a toilet in the north eastern corner of the bathroom.