From GameWatcher: “Call of Cthulhu is out now, and while it has some problems it’s still a worthy adventure-RPG with a fair number of cool puzzles. It’s not essential for progress, but it is a confusing one – the Call of Cthulhu Piano Puzzle.
The playable piano is found in Chapter 6, and while the piano isn’t key to winning, it’s still got a reward – so let us show you how to solve the Sanders Residence Call of Cthulhu Piano Puzzle and what you get for it!”
Source: N4G PC Call of Cthulhu Piano Puzzle – Sanders Residence Piano Puzzle Solution