I use Movie Edit Pro, I want to fade an audio track to a lower volume without muting the track. I currently achieve this by copying the track to a new track, lower the volume to a level I want and then fading the 1st track to mute.
Don't you know the Group/Ungroup facilities ? Mark the video/audio tracks, and click Ungroup over the Timeline (a broken chain) (or press Ctrl+M). Now you can fade the Audio volume, without changing the video track. Another way is to use the Drawing mouse mode (Alt+3) and draw the sound curve. (MEP 14+)