Wouldn't reducing the volume of the other streams in x db below the notification be much easier to implement and achieve the goal of hearinf the notification? X could be different based on the urgency of the notification.
The problem is that if those other streams are much louder than the notifications, reducing their volume won't be enough. You will only hear a strange gap with volume going down and up, but you may not hear the notification in the middle of the gap. If we want notifications to be audible, their volume must be matched to that of whatever is currently playing.