We have a Audio Volume (from plasma-pa) and a Audio and Video (from plasma-desktop/phonon) KCM with overlapping purpose. Those should be merged somehow.
The plasma-pa UI is more modern. It's QML based whereas the Phonon KCM is still using QWidgets.
The Audio Hardware Setup tab from Phonon KCM is duplicate to the plasma-pa KCM. Only the Speaker Testing section is unique. That should be moved to plasma-pa.
The other two tabs (Device preference and Backend selection) are Phonon-specific. I'm unsure if they are widely used and worth keeping.