You can take the mouse pointer at the end of the audio file (kinda tricky,
you have to get the pointer to change to <-->) and then you can drag the end
of the sound file to where your video ends and then tell the audio to fade
out. The song isn't going to be finished, but it will fade out when your
video/pictures end.
Jeffrey's method will work but if you only want to fade out the audio
temporarily the easiest way is to split the clip in 2 places. 1) Where you
want the audio to fade out. 2) Where you want the audio to come back.
Then you can fade out the audio at the end of the 1st clip and fade it back
in at the beginning of the 3rd clip.
