Okay, after a lot of coding with my buddy Chat GPT, I made some progress. You can check the code here https://github.com/campidelli/arduino-accordion

I am using OOP because, as a Java developer, it is much easier to understand what is happening.

Here is the project structure so far:

This could be used for any virtual instrument, in this case, I am making an Accordion, but it could be a Marimba, Drums, Piano, etc.

The instrument gets one or more Inputs (I am using one named PianoKeyboard) and one or more outputs (I am printing the Notes/Chords to the terminal).

An output example here:

Because I am using the Keypad library, I can simultaneously press up to 10 keys (https://github.com/Chris--A/Keypad/blob/master/examples/MultiKey/MultiKey.ino)

 The next stop is to generate some actual sounds!