Suppose this parallel plate capacitor is connected to a 9-volt battery. A volt is a measure of the electric potential difference. In short, this is the electric potential energy per charge — it’s a measure of how much energy a charge would gain by moving across that potential. So, this 9-volt battery will create a 9-volt change in potential across the plates.
But what would happen if you pushed one of the plates so that the distance between them decreases just a little bit? Well, since the capacitor is still connected to the 9-volt battery, then the potential would still need to be 9 volts. However, if the electric field stays the same, a shorter distance would mean a lower potential. The only way to compensate for the decreased spacing would be to increase the charge on the plates. This extra charge would come from the battery and it would look like an electrical current. On the other hand, if you move the plates farther apart, then the charge would come off the capacitor and also produce an electric current.
In other words, moving the plates back and forth creates a changing electric current. This is the basis of how a condenser microphone works. When you have a sound, it produces oscillations in the air. These oscillations then push on one of the plates of the condenser microphone to create a changing electric current. You can then record this current and save it for later, and you can send it to an amplifier and speaker to produce louder sounds.
The nice thing about a condenser mic is that one of the capacitor plates can be very thin and flexible. This means that it can move quite quickly in response to higher-frequency sounds, so you might not be surprised that many high-end microphones are of this type. Of course, one small downside is that these microphones need an applied voltage, meaning they need a power source. This could be from a small battery in the microphone or, more likely, power supplied from the audio receiver / amplifier.
Now let’s look at a slightly different kind of microphone: the electret mic, which is sometimes called an electret condenser microphone. What the heck is an electret? The name should remind you of something familiar: a magnet. Although it is possible to create a magnetic field with an electric current (like with an electromagnet, as demonstrated here by Wile E. Coyote), most people probably think about something like a permanent bar magnet. These are made from materials that have tiny regions that also create magnetic fields called magnetic domains. When these magnetic domains are aligned in the same direction, you get a magnet with a north and south pole.
Instead of having permanent north and south poles to create a magnetic field, an electret makes an electric field using positive and negative electric charges. It’s sort of like when a sock comes out of the dryer with a static electric charge and sticks to stuff. (Well, a sock doesn’t stay charged, but an electret does.) While a sock might just have an excess negative charge due to some extra electrons — or a positive charge due to missing electrons — an electret can actually be neutral. Even if an object has an equal number of positive and negative charges, it can still make an electric field if there is a “charge separation.” Imagine a molecule with one side that’s slightly positive and the other side that’s negative. It will still be neutral, but it will create an electric field.