is there a way to make this buzzer quieter? would you use a large resistor?
Yes, I would put a potentiometer (maybe a 5k - 20k ohm or larger) in series with the piezo and increase the resistance until the desired sound output is reached. Then remove the pot from the circuit and measure it's resistance, and replace it with an approximately close value discrete resistor.
The mounting scheme of the piezo will also have an effect on the sound amplitude. If the piezo is screwed down to a plate... it will vibrate the entire surface and will gain amplification. Stick it to some foam tape and it will be quieter. Just something to be aware of.