When applications are playing audio, there is no entries for it in the kmix window, only the hardware mixer is shown and none of the settings tabs show any option to put in app volume sliders Reproducible: Always
Reported for old version. KMix does show playback streams for applications that provide them either through PulseAudio or MPRIS2. If this is still a problem with up to date KMix and Plasma 5 then please reopen and confirm which applications are not showing playback streams.