How to Double the Sound From Your Buzzer

The piezo buzzer reacts to the magnitude of the change in voltage across its terminals.
You can double the volume of the sound created by doubling the voltage change.

To do that, instead of attaching one pin from the buzzer to GND (where it is permanently 0V),

Then, modify your sketch so that immediately after it drives the first pin HIGH, it
drives the second pin LOW, and vice versa.