If you want audio to automatically switch to HDMI when plugging in a new device, do the following:
Once again, the proof that KDE software can do at least as much as Windows...
The Plasma workspace allows users to create customized themes whose widgets can be themed using multiple themes. For example, it is possible to have the Taskbar be themed using Oxygen, to have the clock be themed using Aya, and much more customization, allowing you to pick the best plasma themes for each object.
Here you can choose what theme is used for different objects in the Plasma workspace. Configure them to your pleasure (it often helps to test each individual plasma element by applying the entire theme to your desktop first) and press.
At this point, the workspace settings dialog should list a Plasma theme entitled (Customized). This is the theme mashup that you have created. Select it and hit.