To start this quest, find Agatha's house, located NE of Meresti trainyard and west of the Scrapyard. After taking to Agatha, she will ask you to retrieve the Soil Stradivarius from Vault 92, right at the very top of the Capitol Wasteland in the eastern corner.
Once you have reached Vault 92, proceed to the sound testing area where the the Soil Stradivarius is located in one of the performance rooms, dealing with a few mirelurks on the way. Then Simply leave the vault and return to Agatha, who will reward you with the frequency for her radio music.
You can alternatively take the Soil Stradivarius to either Abraham Washington in Rivet City or Ahzrukhal at The Ninth Circle in Underworld, who will both pay you 200-300 caps for it. No matter who you give it to, the trophy will unlock.