Select the audio layer, or video layer that contains audio, and hit the L key twice in succession. This will reveal the volume property and the audio wave. Set a keyframe where you want the fade to begin, then scrub forward to where you want the audio to be off and set the volume to a very low value, e.g. -50. Select this last keyframe and hit F9 to make it easy-ease and fade the audio out more smoothly.
You may already know this, but you should ensure that any audio you use in AE is uncompressed (no MP3s) otherwise you will likely have problems with your render.
Simon Bonner
