A piezo buzzer contains electronics to make it buzz.
You have a piezo transducer.
You can make it louder:
- Place the piezo in a case, like a loudspeaker box. That makes it a lot louder.
- Use two alternating pins in anti-phase as MarkT wrote (and use the resistor he mentioned).
- As last option, you can use 12V. But you have to use an output stage. You can build a push-pull output stage with transistors (H-bridge). It can also be a power-mosfet driver if the frequency is not too high. A piezo could have a capacitance of 30nF, but I have one that is 100nF. For high (ultrasonic) frequencies you can't just have a square wave to drive the piezo, you need a sine wave.