Currently, the mixer provides only two kinds of notifications when mixer track levels are changed: the tooltip (in case the mouse is on top of the panel plugin icon) and the changing icons on the panel. 

In some cases (for example, when the user wants to know the exact change in percentage but the mouse is not on top of the panel icon), it would be handy for the end-user to have a libnotify visual confirmation of the changes. 

Should this be implemented, it could be extended to all the other controls, that is a notification will be popped-up whenever the level/state of a control is changed within the mixer. At the moment there is no kind of notification for these changes.