Decrease apartment intercom door buzzer volume
you need to reduce power to the diaphram. A fixed value resistor is a decent way of doing that.

Cut a white speaker wire, patch it with a resistor, which you can get at radio shack or wherever. Start with matching the speaker ohms (likely 4, 8, or 16) to ~half the volume. a pack of 5 5-10ohms would be good because you can connect them in series to get multiples of 5 or 10 to adjust the volume. 

you could replace the speaker with a higher-ohm model as well, they make them in all sizes, so i'm sure you could find a drop-in physical replacement.

A potentiometer might work, but has some complications: most are too high resistance (ex 0-100 ohm), most zeros aren't zero, but rather 5-10 ohms, which steps on your adjustment range a lot. Many pots don't handle very much current either, so you could fry a cheap/small potentiometer, although that's probably not likely.

if you don't want to cut wire, you can "short" the speaker with a resistor, instead of wiring it in series. this reduces the amount or power going to the diaphragm, but wastes power. don't put fewer than 10ohm across if you go that route.