Here’s one way without parent child, and using two tracks.
Top track – boy playing piano.
Bottom Track – Music sheet.
Open Pan Crop on top track, click Mask at the bottom and create a bezier mask around the boy, with feathered edge. If you split this Event where you want to make the change, then turn off the mask for the left hand portion, the edge of one side of the split can dragged over the other to create a fade in the music, and the same later the other way, to fade out the music.
Peter Wright
Perth, Western Oz
http://www.allroundvision.com.au