Audio Levels to Keyframes

You can use audio levels (volume) to control other properties, via expressions, but first you must convert audio levels to keyframes, or rather let After Effects do it for you.

Select the audio layer and choose Animation>Keyframe Assistants>Convert Audio to Keyframes. After Effects will add a Null layer (named Audio Amplitude) to comp. Select this layer and press U on your keyboard. The layer will twirl open , revealing three effect properties ( assuming you're using stereo sound): Left Channel Slider, Right Channel Slider, and Both Channels Slider. You can use these properties as controllers for expressions.

If you add a small Solid to your comp and add an expression to its Scale property, you can then Pickwhip the Both Channels Slider property of the Null layer. As the music gets louder, the Solid will grow; as the music gets quieter, the Solid will shrink.

If the Solid doesn't grow big enough to suit your tastes (maybe because the audio never gets very loud) try adding a * 2 to the end of the expression. This means "times two," and it multiplies the expression by two, doubling the scale changes. If the solid still doesn't grow enough, change the 2 to a larger number.