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

hm, you do realise you are using a really old version, don't you?
Anyway, in the volume settings (click on the speaker icon in the panel bottom right), you can choose to disable specific channels, specific applications or mute the complete output. You probably have not muted the output but specific channels.

I hope this shows as it does for me, but I am several versions ahead of yours.