Instructions
The Paint Route
Approach the organ and it will give you a chance to finish its song. If you wish to let the ghosts back into the manor, you will need to take this route.
Jump on each painted key just after the gold notes appear above it. Move quickly or you will have to start over. You will also have to start over if you jump on the wrong key.
Complete the song to let the ghosts back into Lonesome Manor. You will receive the "Play a Tune" pin and be able to use the walkway on the organ's pipes.
Thin out the walls near the organ to find a gremlin trapped in a cage and the pressure switches you will need to activate to proceed.
The Thinner Route
Approach the organ and it will ask you to complete its song. Instead, begin to erase the painted keys (they will be brighter than the rest).
Finish erasing the keys and the enraged pipe organ will send two beetleworx to attack you. Thin out their armor and use your spin move on their backs to defeat them.
Thin out the walls so you can continue. By choosing this route, you will get e-tickets, but will not be able to proceed to the pipes behind the organ. You will not get the pin and you will fail the ghost's side-quest.