You can record keyframes on the audio track in the scene director. There is a little arrow to expand the audio track and a little red "record" button that marks a keyframe. You can then adjust the volume of the audio track to fade it out. However, it is far easier to actually edit the wav file so if you need to use it in multiple scenes you don't have to keep adjusting the scene directors.
There is a free WAV editing application called Audacity that works very well.
#XPression