The PDF manual states that you can double click within an audio object in the timeline view to set an inflection point for the volume vs. time curve. However, no matter where I double click within the audio object, I just get a popup window called "Sound optimization." The popup window has tabs for Noise removal, Equalizer, Compressor and Stereo FX and controls for DeClipper, DeNoiser and DeHisser, but no inflection point is added to the volume curve and there's no way to cause a simple fade.
I finally figured out that I could get a simple fade by playing with the four predefined inflection points on the volume curve, but adding inflection points does not work.