To acquire this quest, speak with Agatha at her house.She requests that you retrieve a rare violin for her from Vault 92. You can either go to Vault-Tec HQ (she gives you this location) or just head right to Vault 92 (check the link for its location). Either way, you will need to enter the vault and find the Sound Testing section. The violin will be laying on a table in one of the testing rooms. See the link below for directions inside the vault. Once you have the violin, return it to Agatha to finish the quest.
See Agatha's Song for more specific info.