This assistant is new to AE6. Previously you could use motion math or a third party plug-in like Trapcode's Sound Keys, but now, using audio amplitude to control other properties is native.
Make sure you have at least one layer with audio.
Set your work area to the length you want to generate keyframes.
Choose Animation>Keyframe Assistant>Convert Audio to Keyframes.
A new layer will be generated called Audio Amplitude.
Select this layer and press U to reveal animating properties.
After you reveal animated or modified properties, you'll see the assistant has created amplitude keyframes for the Left, Right, and Both Channels of the audio layer (or layers ). Using a true stereo file will produce the best results as the left and right channels will vary. If you are getting the same amplitude levels for both left and right channels, you could disable them (by hitting the stopwatch) and just use Both Channels' keyframes.
Now you can take a property of any layer or effect applied to that layer and use an expression to link the property to the audio amplitude. Amplitude tends to be low so you should add a multiplier at the end of your expression-like this Comp layer ("Audio Amplitude").effect("Right Channel")("Slider")*10.