This achievement is for completion of the 'Agathas song' quest
The quest requires you to enter vault 92 and find her great-great-grandmother Hilda's Soil Stradivarius violin.
Agatha is located in her house to the Northeast of Meresti Trainyard.
The vault is located at the top of the map, west of the Republic of Dave and east of the Oasis.
Once you enter the vault, go straight through the door, until you get to the open room and go to the left. Following this path go to the far side of the next room and down the stairs to enter sound testing.
In the sound testing area, go straight ahead and follow the corridor along to the right, take two lefts and proceed down another set of stairs. Take the next left to find two rooms. Go to the computer in the room beside the studio and unlock the door, then into the studio to get the Soil Stradivarius. Finally, take it to Agatha and hand it in for the achievement.
[You may also ant to pick up the sheet music, in the men's toilets in the vault, for the Blackhawk Magnum from Agatha.]
You can get the violin before talking to Agatha to avoid two trips to her house.
Alternatively you can sell it to Abraham Washington at Rivet City for caps and the achievement.