The article in Popular Electronics discusses how there are two ways to make this circuit. You can test whether the comparator is zero by using a galvanometer (or potentiometer) if you give the circuit a voltage. However, if you feed it an audio signal, you can use headphones to hear when the sound is silent. Most of them used the meter, but some like the G.E. kit used headphones.

I am going to use both. I want it to be relatively cheap overall, so I don't want to embed a meter into the computer. I'm going to have two posts in order to attach a potentiometer, but I'll also have audio input/output jacks. A simple switch will select which to use- so the audio option is essentially off.

Here's the version with the meter.

Here's the version with headphones.

You can see that they're essentially the same circuit, so both can be wired with a single switch. These images come from the Popular Electronics article.