Take the plastic box off the music box element.  Wind up the element and hold it in the air while it is playing.  The sound should be just barely audible.  Attach the plastic box to the element, or touch the element to a table top or tuning fork box.  The sound become very audible with the addition of the chosen sound board.

The front and back of a miniature piano sound board are shown with this demo.  Tapping on various places on the sound board will reveal those spots with good resonance.