It seems to me that this problem is related to the flat-volumes issue already reported in the bug #541538 [1].
Time ago I solved this problem in this way: > echo 'flat-volumes = no' >> ~/.config/pulse/daemon.conf > pulseaudio -k && pulseaudio --start However, although in Jessie and Stretch flat-volumes are still enabled by default (the pulseaudio manual and /etc/pulseaudio/daemon.conf say so) and removing the modification above, I do not experience the issue anymore on different machines with Gnome and Plasma 5. But I wonder why it is no more an issue, at least for me. Are flat-volumes managed in a different manner now? Do you have any clue about it? Regards Francesco [1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=541538