I have a very small speaker salvaged from a cellphone, and I'm wondering if it can be directly connected to Arduino pins in place of a piezo buzzer.
I'd start by connecting one end to ground and the other to an Arduino output pin through an electrolytic capacitor of 100uF or more in series with a 150 ohm resistor. If it's too quiet, you can reduce the value of the resistor, but you should keep the value of the resistor not below 120 ohms minus the resistance of the speaker.