In the system tray kmix dropdown, there is no option to display playback sources instead of playback devices in the menu. Reproducible: Always
Do you mean that it doesn't show individual applications? If yes, I'm voting for this to be fixed. Going to "Audio Volume Settings" is very inconvenient, when all I want to do is to change the volume of one application.
Actually, I was using plasma-pa instead of kmix. Kmix works fine for me so I'll remove my votes.
Definitely supported, both with PulseAudio and without. You may need to enable the showing of "Playback Streams" in Configure KMix - Volume Control. However, not everything may work properly. Some applications - notably Firefox - don't support MPRIS2 and so will not be shown without PulseAudio. Chrome is a mess, it advertises but doesn't respond to MPRIS2 and so will show volume and media controls which don't work. The same applies to any WebEngine-based browser.