Agatha's House is located somewhere to the East of the Meresti trainyard. Her home-made violin was quite helpful in entertaining the trade caravans, but it's having trouble with the tuning.
She knows the of a rare Soil Stradivarius in Vault 97, but it's location is unknown - you need to enter Vault-Tec headquarters to determine it's location.
Locating the vault
You will discover the location of the Vault during the Picking up the Trail quest and view the map of the vaults. You also have the option to go to Vault-Tec Headquarters, which has an incomplete list of the scattered vaults. Otherwise, you may have to explore or obtain it's location through other means.
Vault 97
Vault 97 was officially declared to be the place where music and artwork shall be preserved. As you approach the vault, you will notice that the door was opened for quite some time and that it is infested. You will also find audio logs of a vault resident who was trying to figure out what was going on.
There are five major sections, including the main atrium. However, you will notice that there are live frag mines placed within the complex, as well as makeshift traps from shotguns and tripwires.
Reactor level
The reactor level of the vault is flooded. Aside from a few mirelurks, this section is not too difficult.
Living quarters
The living quarters are infested with Mirelurks. However, they can be instantly killed by using one of the terminals to create an ELF noise, which causes their heads to explode due to the low frequency; this requires Science 50 or the password to access the terminals.
You can unlock the data storage area with a lockpick 100 or science 75. This small room contains various supplies and ammo.
Recording Studio
The recording studio doesn't have any sound flush system, thus you will have to engage or sneak past the mirelurks. As you may noticed in the other sections of the vault, they can attack in force.
You should find the Soil Stratesvarius in the lower floor of the studio.
Overseer's office
If you want to know what happened with the Vault, you can read the Overseer's terminal. If you have a low science skill, you can also obtain the password to the system.
Return to Agatha
Once you retrieve the violin, you can return to Agatha. She can't give you anything, but giving the violin allows Agatha to broadcast her music on the radio tower.
Collecting music sheets
While not required to complete the quest, you can talk to Agatha more to convince her to record music on paper. Agatha will ask you to find a music sheet for her. There are several scattered around the wasteland, one of which is in Roosevelt Academy.