The Naze32 Flight Controller has a “lost model beeper” output, which looks like this on the schematic diagram:
We can see the beeper output is an open collector transistor, pulling one side of the piezo down to ground.
The other side of the piezo goes to +5V.
Check the specs of your piezo buzzer.
This is the piezo I am using from Altonics, which is already louder than a lost of normal piezos (as previously posted about here):
If the piezo can tolerate a higher voltage, then it’s very easy to make it louder by removing the piezo’s +ve wire from the Naze32 board, and taking that to the main lipo battery +ve feed, like so:
Here is a video that demonstrates the difference between a piezo running off 5V and 3S lipo voltage.
All you need to do is move one wire, and the piezo is much louder.
Pick the most convenient point. Here I use the +ve feed for the nearest ESC:
Solder pads are covered with liquid electrical tape to prevent accidental shorts.