Who Knocked Out The Howler Monkey?

The Howling Monkey gets knocked on the head in the middle of the night while he's singing (actually, more like howling). He's lost his mind and now hums nice romantic tunes. Who did this? The Beaver? The Koala or the Boa? Meanwhile, Hermione asks Sherlock to find another instrument to play instead of his bagpipes. Not so easy.