How can I reduce the volume of my dryer buzzer?

Got a brilliant idea from one of the EE's at work... iPhone wall adapter as AC/DC converter. Plug tongs fit inside the clips that connected the previous buzzer. USB cable cut and wired to Piezo. The iPhone adapter outputs DC, drops the voltage to 5V which the Piezo can handle, and it work like a charm. My wife can't understand how I did it. I'm a champ (for a day).

As Sunny said, tape over the hole is the easiest option.

The next thing I would try is adding a power resistor in series with the buzzer. The issue is, you need to know approximately how much current the buzzer draws.

I found a couple online, and it looks like 5-15W is a safe bet.

https://www.searspartsdirect.com/product/1ev8sv2nmu-0026-110

Wow that's a lot, anyway...

Now, I'm not 100% sure here, so anyone please correct me if I'm wrong, but according to this source, you need to reduce the wattage to approximately 1/100th to perceive 1/4 the loudness.

That means you want to reduce 10W to 0.1W, maybe the buzzer won't even work at that level.. but lets continue. The formula is W = V^2/R, R is fixed (that's the buzzer impedance), and we know we want W to be 1/100th of what it was, so that means V will be reduced to 1/10th or 12V. The impedance of the buzzer is 120^2/10 = 1440 ohms. At 12V we'll be pulling 12/1440 = 0.0083A. We need to drop 108 volts, so 108/.0083 = 13k. And the wattage will be 108*.0083 = 0.896W

So TLDR; I'd try a 10k 2W power resistor in series and see what that does.