KDE applications may play back sounds to draw attention to application state, warnings, errors and other wortwhile notifications. Play back of sound is not always desirable, hence the need to be able to mute all notification play back.

Explicit configuration of "muting/unmuting" all the system bells/whistles/jingles and beeps has been removed from KDE 5 notification systemsettings. Judging by the KDE forums thread, this functionality is supposedly provided by the magic of pulseaudio and controlling volume on the "Event Sounds" audio output stream, now.

Unfortunately, in reality, applications using phonon do not route/mix their beeps and bleeps to /into this "Event Sounds" stream. Instead each application plays back sounds using their own (transient) output playback stream: circumventing and defeating all attempts at configuration through KMix 5.

Reproducible: Always

Steps to Reproduce:
0. Read: https://forum.kde.org/viewtopic.php?f=289&t=126735#p336439
 - Note, apparently notification sounds are supposed to be mixed into the "Event Sounds" stream
1. Set up a way to see/screen cap the problem:
 - Arrange for a pavucontrol window side by side with an application that exhibits the bug
 (- Set up ksnapshot or to screencap a region with a delay)
2. Trigger a notification sound:
 - For example: navigate away from a system settings module with unsaved changes 
3. Observe in pavucontrol that the warning sound is played back using its own (application) output, instead of being mixed into "Event Sounds"


Actual Results:  
The Event Sounds stream is not used, instead another output stream is created and used to play back sound -- by passing any desired volume control on the part of KMix entirely.

Expected Results:  
I'd hoped phonon would mix the notification sound into the "Event Sounds".

In KDE4 days the problem may or may not have been present, but at least it could be globally 'fixed' by the expedient of telling the notifications KCM to never, ever, produce any sound output at all -- which was a quick and easy (albeit perhaps obscure) way to silence all the bleeping/jingling bells.

Since, apparently in KDE 5 the expected behaviour under pulseaudio is that KMix takes care of it through muting/unmuting the "Event Sounds" stream, phonon must route all this stuff automatically through to the proper output sink for the feature to work correctly.

However, clearly, phonon is not performing this task properly, introducing a rather major regression in functionality to KDE 5.