Problems about Serial.println

Hi! I recently made an Arduino music Keyboard project. I first finished the C scale. But when I tried to add a button that when pressed, convert the C scale to B flat scale. However, the C went immediately off tune. But everything else is fine. I tried to reprogram each part to locate the error, it seemed to be the "Serial.println.." that affected the output! But aren't Serial.println just the code that prints stuff to the serial monitor? I don't know what's going on, need your help! I have screenshots of my program(with and without the minorKey button) below.

I have screenshots of my program

Well I'm not going to waste mobile data on 700kB of image.
Just post your code.
In code tags.

You need to post your code where it can be read and copied and worked with. Why in the world would you post pictures of the code instead of the actual code.

I suspect the wiring.