The audio fade in/fade out behavior works in the same way as with video. Click on the audio tab in the project pane and select your audio. Apply the fade behavior. In the inspector click on the behavior tab and you will see the controls. They are calibrated in frames. You will also see the controls and a graph in the Hud. Go to the timeline editor and open the keyframe editor and you will see the level curve. Double clicking on it will add keyframes and you can keyframe the levels as you like. If you don't want to use the behavior just open the keyframe editor after selecting the audio in the timeline editor or the project pane. Then as above, keyframe the levels as you like.