This is unlocked in the Basement boss fight with Baby Plum. When the fight starts, DO NOT attack her. Just dodge her attacks for a while, and eventually she will wave goodbye and fly off screen, leaving the Plum Flute on a pedestal to pick up. If you do accidentally shoot, or you have blue flies that attack her, it's still ok. You just have to go about 20-30 seconds without dealing damage and she'll leave.