When you get upstairs the horn is hanging from a single nail.
Judging by the bat droppings the bats use it to roost there. But, at the moment, the bats are flying.
This is the low-spoiler guide for "Getting the horn" puzzle in Lucy Dreaming. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.