each of the instruments and you can pick it up.
to get the first hidden piece, plug in the cord on the far left wall, then turn on the switch on the display case. the piece is on the green tile on the bottom shelf.
the second hidden piece will appear on the music sheets on the display case next to the painting. it will appear after you move the instruments on the desk.
the third hidden piece will appear after you turn the crank several times on the standing music box.
to get the 4th hidden piece, look for the horn on the central table. first touch the name plate, then remove the 