Using light to control the 'fades'/volume of each channel by using a LED and a photoresistor. You can simply control the LED with an Arduino. I know, a photoresistor is slower than a photodiode, but maybe it makes it more smooth and avoids hearable digital steps. It feels like analog and circuits are completely separated.
I made some drafts/examples at my question, my question is about if there is component that combines the two. There is already an answer that mention some partnumbers. More info and picures at my question.