How to add an UPS beeper volume controller so that I can make it sound quieter when it beeps?
That's a common self osscilating Piezoelectric buzzer. You can lower the volume electronically by reducing the voltage to it. A few 1n4001 diodes in series would work. A low ohm resister in a pinch. Or you can just snip it off, or pierce the membrane inside with a screwdriver 

But a mechanical method works fine too. Some tape over the hole, or some cotton in the hole. Both. Muffles the sound.