Hello again,
They also make a little component that is made just for audio purposes. It is a small package with a single small LED and photo resistor in a light tight package.
When the LED has current flow, it shines on the photo resistor and the photo resistor decreases in resistance. The two leads for the photo resistor come out one side and the LED leads come out the other side of the package. The two resistor leads are used for the resistor to set the gain for the amplifier. This creates either a compressor or an expander depending on how you condition the audio signal to drive the LED. The resistor is of course linear for the most part and works with bipolar signal levels.