So you're trying to push a whole bunch of buttons and get multiple frequencies right? That's 'polyphony'.
I don't think he is. What that circuit is designed to do is to detect how many buttons are being held down at one time. He said
Then, I'm trying to add some new frequencies (notes) pushing together 2 or more buttons, producing different values (in order to collocate a new frequence for each new value).
As I read that there is only one tone being generated at any one time.