Reproduced by installing a fresh version of KDE neon User Edition then attempting to change the volume (by using volume keys and middle mouse wheel). Tested on a fresh install of KDE neon and an up to date install. Fixed by: `sudo apt install gstreamer1.0-pulseaudio phonon-backend-gstreamer-common phonon-backend-gstreamer libcanberra-pulse` These packages are missing from the KDE neon iso, not sure if it's fixed in the dev editions.
It's December and this is still an issue.. Would be good to fix this since it is a very easy fix, already provided by the author and works in the latest Neon user edition as well..
I can confirm this error in KDE Neon User Edition LTS. My default phonon backend is VLC. At this point in the system-settings tab audio hardware configuration, the left/right sound test don't work. Solution proposed by monsterjamp23 works. After install those packages, the sound test works.
Problem also noted on Neon Dev unstable Bionic (p 5.13.80, fw 5.50.0, qt 5.11.1). Installing libcanberra-pulse enables sound on my system.
*** Bug 399521 has been marked as a duplicate of this bug. ***
This was working fine for me in 5.13.5 and then, after upgrading to 5.14.0 it stopped wroking. I can confirm the solution provided in the first comment works, however, I had to configure the volume applet to unmark the "Volume feedback", save it, and then configure it again, mark the "Volume feedback" checkbox, and save. After this, I get the Volume feedback working when changing the volume