I'm not quite sure what you mean by saying that the door buzzer is "in the plug box". Are you saying that the buzzer itself is in a junction box that also contains an electrical receptical? Would you have a picture of that?

Door buzzers and bells are pretty simple. Generally, all that is involved is a small transformer (to provide lower voltage power), a switch, and a bell or buzzer. All of these things are connected together with some relative small wire, since there's not much current involved. The wire itself probably doesn't need to be replaced unless the insulation is cracked, or there's some other flaw. The devices (transformer, switch, and bell/chime/buzzer) can all be easily be found at any home improved store. I usually replace all three of these items at the same time, particularly if they've been around for a while as yours have.

Of course, if the wire is accessible, you can replace that as well. Nowadays, lots of folks go with wireless devices to skip the challenge of hiding wires behind walls.

And please turn the power off to the circuit breaker or fuse supplying power to the door buzzer before you attempt any modifications. Even though the voltage is relatively low (except for the 120V supplying the transformer), it can still knock you off a ladder if you get in its way.

Yep, it is located in a junction box designed for (2) dual plug outlets. 1/2 has the plug for the fridge and the other 1/2 contains the door buzzer. Did I say the buzzer sounds like hell! Even the restoration hardware stores in the area have never seen anything like it. Maybe I will just disconnect it and install a pretty powder coated brass knocker on the front door!

Doesn't sound like anything I've ever seen. I'd love to see a photo of it, if you can post one somewhere.