At the moment media controls widgets in systray renders dropdown menu to select the audio source that should be used to present data for. My suggestion is it replace this dropdown menu with by adding footer section where single icon will be rendered as a combobox to select source and the selected source name will be rendered as a static text next to this icon. I have attached image that shows the concept.
Created attachment 129315 [details] Audio applet before changes.
Created attachment 129316 [details] Audio applet after changes.
Oh, that's a great idea!
We ended up using a tab bar in the Plasma 5.21 redesign.