You should have some spare pins left on the arduino.
So i can just add normal buttons to a digital pin? This shouldn't then be a problem, at the moment i am using only 3 of the digital pins.
With the LEDs the simplest from a programming point of view is to add a shift register:-
for the LEDs, i have a couple of 74HC595N. I saw http://arduino.cc/en/Tutorial/ShiftOut that we can use it to plug several LEDs. But i would have really NO IDEA on how to right a code reading which piezo was played and telling which LED should light up!
Well, i think i will start with the buttons. Maybe that is something accessible for my level.
Anyway, i don't have anymore multiplexers, so i need to wait until tuesday when i'm going to town to buy one more.