As I said before, I would add, for example, the note "sol" pushing for example button 3 and button 4 at the same time, but the value shown on the screen (thanks to the Serial.println) is the same of that of button 3, so it's impossible for me to associate a new value to the frequency of the note sol.
Before we go any further, I think we need to know where you stand with regard to electronics. This is the General Electronics Topic forum. You are asking questions about a circuit which is a simple voltage divider. We need to know if you actually KNOW what a voltage divider is and if you are familiar with ohm' Law. For example , you ask a question about pushing buttons 3 and 4 together. Button 3 is 10k and button 4 is 1 Meg. How much effect do you think a 1 Meg ohm resistor is going to have in parallel with a 10 k ohm resistor ? Do you see my point ? Have you done the math for each button ? (the voltage divider math) Do you KNOW how to do the math for the voltage dividers ? Have you Googled voltage dividers ? Do you know how to do the ADC conversions ? Do you know ANYTHING about HOW this circuit works ?