Suppose many many others does manage this problem.
On Kubuntu 18.04 LTS with all latest updates I can year videos on YT or Vimeo but voices are peaked and almost not noticeable. Standard audio can be heared even if not well.

I have no speakers associated with the display, so I need to use the mobo (ASUS Prime B450-PLUS with Ryzen CPU) output trough the audio jack on the case.

Tried to adjust Audio Playback Device preference so to set this as priority audio output.
Tried disabling every other audio hardware (having two GPU each other with his audio output my concern was might create some conflict).
Tried setting the audio volume below 100% for all the applications.
Tried to follow this guide to update the daemon.conf.
Tried using Chrome instead of Firefox.
The listed backend is Phonon GStreamer 4.9.0.

Curiously tying to download a mp3 file and listening it using VLC or Cantata I can't hear anything at all.

Any suggestion is welcome.

I would test with a Live CD of something other than a *buntu and a different set of known-good speakers, if you haven't already done so.

If the problem persists, it may be a hardware problem. The most likely culprit may be the jack itself.

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