In timeline view, go to the audio mixing room (the treble clef sign). In the timeline, click on the clip you want to fade out and move the timeline slider to about 3 seconds from the end of the clip. Then, in the audio mixing room, click on the fade out button (little speaker icon) at the bottom of the slider of the appropriate track. Another way is to "grab" the volume line in the track in the timeline, but the first way is more automatic.
Michael