Agatha's Song

 is a Fallout 3 quest. It is also an Xbox 360 and PC achievement.

Walkthrough
Agatha is an old woman living alone in a house in the Capital Wasteland. She is a talented violin player who broadcasts her music through her late husband's old radio gear. She wants the Vault 101 dweller to find music paper for her, as well as her great-great-grandmother Hilda's Soil Stradivarius violin, which she believes is still in Vault 92. (Optional) You can visit Vault-Tec's Headquaters to get the location and passcodes for Vault 92.

Vault 92 itself is infested with Mirelurk, Mirelurk Hunter, Bloatfly, and Radroach. The Vault is decently easy to navigate, so long as you follow the signs, you can explore the Vault to find various items including bullets, parts to assemble weapons, and varies drugs and aid items. To get the Soil Stradivarius, the easiest way is, once you enter the vault go through the door straight ahead of you, follow the corridor until you get to the open room with the Bloatfly, take the left most doorway and follow that corridor until you find the Mirelurk Hunter, there will be 2 in this area. You'll have to find a switch to open the studio room, which you will have to fight both Mirelurk Hunters to get to, once you find the switch, go into the studio to get the Soil Stradivarius and take it to Agatha, she rewards you with the frequency to her radio station.

to sum it up: Its in the "Sound testing" Area follow the LAB sign to get down the stairs and you will find it sitting on a table in the music testing room. If you continue to talk to her, she asks you to find her some music paper.

The sheet music is located somewhere on the floor of the Arlington Public Library.