SUMMARY
Usually I have connected to my PC more than one audio devices (speaker, headset, etc.). In plasmoid "Audio Volumne" is present "Applications" section and here I cannot switch audio output for any application playing currently some sound to any other audio device than currently playing.
"Applications" section shows list of applications playing currently sound where each of one has option: "Show additional options for ..NAME_OF_SELECTED_APPLICATION", but clicking into it does nothing. Only this button looks like to be held.

Please notice this was possible in recent version of Plasma, so in 5.19.5.
It looks like regression.

STEPS TO REPRODUCE
1. Connect to PC at least 2 audio devices
2. Play some sound in two applications, for example in audioplayer and webbrowser
3. Open "Audio Volume" and click into section "Applications"
4. Try to switch audio output to any audio device (I suppose button "Show additional options ..." should show list of available outputs)

OBSERVED RESULT
Clicking button "Show additional options ..." does nothing.

EXPECTED RESULT
Clicking button "Show additional options ..." should show list of available audio outputs like in Plasma 5.19.5

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: YES
(available in About System)
KDE Plasma Version: 5.20.0
KDE Frameworks Version: 5.75
Qt Version: 5.15.1

ADDITIONAL INFORMATION
I reproduced issue in Arch Linux and in KDE Neon.