Gentoo user, I am using kde-frameworks-5.23.0, plasma-5.6.5, phonon-4.9.0 and phonon-vlc-0.9.0. On a quite default setup, if I do something like "unlock widgets" which will cause a pop up and a notification sound to occur the following happens: 1. sometimes the volume is noticeably louder than when I click the play button in the kcm notifications dialog. This also causes clipping/distortion 2. sometimes the sound is a very garbled loud noise, which doesn't resemble the original sound (in this case Oxygen-Sys-Special.ogg) at all 3. If I click around in the kcm notifications dialog, most sounds seem to have an audible pop at their beginning. if I switch to phonon-gstreamer-4.9.0 backend, the issue seems to go away. Reproducible: Always
the volume difference comes from the sounds not actually being sounds as per stream-category remark in https://community.kde.org/Phonon/Releases/VLC/0.9.0 which is fixed for vlc3 https://quickgit.kde.org/?p=phonon-vlc.git&a=commit&h=001c117c2aa543015140f83184d6fb06728a385f the overall sounds quality issues you'll want to report against vlc directly.