In AE Help, Todd Kopriva lists instructions for the Convert Audio To Keyframes keyframe assistant and some Online resources for converting audio to keyframes.
Among them is a reference to Ducking Audio Expressions posted at Video Copilot forums, meant to automate music fades as voice tracks start:
"Nathan Gambles provides an expression on the Video Copilot website that ducks (reduces the volume of) audio on one layer when the volume of audio on another layer increases. This technique is useful, for example, for automatically decreasing the volume of a soundtrack when dialog occurs. This expression for the Stereo Mixer effect depends on the Convert Audio To Keyframes keyframe assistant having been applied to the other audio layer."
Update: Motion Graphics Exchange listed this last year and has newer related related auto-fading and more.