22nd March 2017, 9:43
I've found a solution. When I've uninstalled the win7 widget, I had to delete the folder .config/pulse while not being logged in (CTRL+F1 at the login screen). After logging into plasma, the volume feedback works again with the old widget. No idea if this can be used to get the volume feedback with the new widget, but I've not seen that option in its settings.