Hey ^densch,
Glad you're enjoying the game and that you're that far into it!
I'll try to give you some hints: Every key in the game is used within Mazen(including Alleyway), so excluding the "levels".
If you replaced the piano key, listen to sound the pianos make and you should be close to solving it.
The note is for something after the piano puzzle.
Let me know how you do:)