I tried to control the volume directly using the MPRIS DBus interface and volume control doesn't work that way either. That indicates that the problem is Chrome's/Firefox's MPRIS implementation. Spotify is known to be broken to us too.

VLC works for me though, so if that works for you too then the problem is most likely not caused by KDE Connect

(In reply to Nicolas Fella from comment #1)
> I tried to control the volume directly using the MPRIS DBus interface and
> volume control doesn't work that way either. That indicates that the problem
> is Chrome's/Firefox's MPRIS implementation. Spotify is known to be broken to
> us too.
> 
> VLC works for me though, so if that works for you too then the problem is
> most likely not caused by KDE Connect


if the problem is with firefox MPRIS DBus interface, why not use kde plasma interface instead? like remotely changing kde menu application/pulseaudio/applications volume? Its a way better approach as it will works with any application regardless of the DBus interface.

btw no browser works I tried even opera

Report for Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1674654

Opera is most likely using the exact same implementation as Chrome.

>if the problem is with firefox MPRIS DBus interface, why not use kde plasma interface instead?

Do you mean going via PulseAudio like the audio applet does? That has the problem that there is likely no trivial way to map from a MPRIS player to a PulseAudio stream.

Besides that it would be in the best interest for everyone if the app's MPRIS interfaces were fixed instead of working around the problem

I guess I wasnt much clear tbh, 

Don't get rid of thje player , but just change settings at system level.
example

When you volume up -> system.application.volume(+10) idk