In the twentieth puzzle of the main game, the one with organ, you are required to play secret 4-tone tune, based on the song from polish metal band Behemoth. Related to the fact that the game narrator is the band frontman.
Press keys number 10, 6, 7, 10. (whites only, from right to left, since left side of the keyboard is smashed)
You may have to try several times, I only succeeded on the third attempt.