When I get a sound notification (for example from KDE Telepathy), KMix reveals that the PulseAudio stream is called "kded5: audio stream" while there is a persistent volume control slider "Event Sounds". Reproducible: Always I used a KF5-based version of KMix built from Git.
Created attachment 92073 [details] screenshot of KMix tray popup
Downstream bug report from another person: https://bugs.archlinux.org/task/44014
Thank you for the bug report. As this report hasn't seen any changes in 5 years or more, we ask if you can please confirm that the issue still persists. If this bug is no longer persisting or relevant please change the status to resolved.