It's a nifty little puzzle to throw at your players. I like Psykie's additional thought of adding a cultist humming the chords, though. Other clues could be spread around the place if need be - old sheet music; the pipes that carry the correct chords could be less rusty; cultists could be interrogated and so on. Go to Comment
Pretty cool, great item for encounters. Another suggestion would be just knowing the cult's ceremonies involved music as well as sheet music if any of the PCs have some musical skill (someone usually plays at least one instrument or can sing). There could also be a synergy bonus for a rogue when combined w/deciphering codes and scripts.