How to make multitone with 8x8 matrix keypad?

Hi!

I have a matrix like keypad without diodes (8x8) and I would like to play with Arduino UNO one tone to 3 or 4 tones simultaneously on one speaker or piezzo buzzer - like chord.

I'm newbie in arduino music and keypads field, so I need a little help.

I have a matrix like keypad without diodes (8x8)

So that will not cope with more than one button being held down at once.

So given that:-

and I would like to play with Arduino UNO one tone to 3 or 4 tones simultaneously on one speaker

This uses a separate output for each tone http://www.jeremyblum.com/2010/09/05/driving-5-speakers-simultaneously-with-an-arduino/

Then you combine those outputs into one with a capacitor on each output, like this:-