You can find capacitors in active electronics circuits dependent on oscillating energy.
Oscillating signals are in electric radios and any kind of electronic audio equipment.
They charge and discharge at immeasurable speeds. In a near instant, they can respond to spikes and dips in electrical charges.
They can produce certain frequencies and help to filter out others.
Signals that oscillate will charge one plate while the other plate will begin to discharge. If the current reverses, the second plate charges while the other discharges.
This allows desired frequencies to flow through while prohibiting the rest.
Higher frequencies will be allowed into the capacitor. Any lower frequencies will be blocked by the capacitor.
The cut-off for frequencies that the capacitor blocks depend on the size of the capacitor itself.
The frequencies that are allowed to pass help achieve the desired sound signal in the end.
Much like a set filter on a mixing board, capacitors can be used to keep frequencies measuring in certain ranges out of the end result.
Depending on the range of frequencies that you're looking for, you'll need the capacitance to respond in kind.