I recently updated to KDE 5.8 and I have this weird and irritating behavior: if I turn the sound off, either as "mute" or lowering to 0, and a window starts playing sound (for example youtube), the volume automatically goes up.

How can I fix that?

Thanks

Kubuntu 16.04, plasma 5.8.5, kde framework 5.28.0, QT 5.6.1

You do realize your "upgrade" is to a distribution release that is 2 years old now, do you? Could well be this behavoir is already gone in the most recent release, which would be Kubuntu 18.04, running Plasma 5.12.5

I am on Bionic and plasma 5.12.5 and for me the same holds: I turn sound of globally, and open a youtube, and then suddenly the sound is on again. (I Even see the red - removed in the icon)

How do you turn sound off exactly? The underlying system is attributing sound by application, and depending on your individual sound card this can be a tad tricky to turn off.
Muting on the icon will not be sufficient, you need to mute the actual output device(s)

having same problem and was wondering same answers. thanks. was also wondering if i can ask questions as i'm new here. Instant Pot Chowder With Clams & Shrimps