I've tried with every resistors I've on the starter kit.. it just work with the 1M ohm resistor.. but just for a millisecond.. while without resistors I still have the same problem.. but the Serial.print write down something strange: constantly 1.63 volts for one Piezo.. and for the other one first 0 and than 5.0 volt... I'm trying to figure it out with potentiometers but still not getting the point..