Dave Ulrick wrote:
It looks like my problem was indeed a pulseAudio feature. In
pavucontrol
I found a slider for "System Sounds" that was set to 90%. (I have no idea
of how it got set that way to begin with; I don't recall ever changing
volumes in pavucontrol.) Increasing it to 100% eliminated the master
volume level changes. Problem solved!
Looks like you're a victim of the PulseAudio "flat volumes"
"feature": set:
flat-volumes = "no"
in /etc/pulse/daemon.conf to disable it.
Setting all volumes to 100% works around that, but the best solution is
probably to disable it entirely.
Kevin Kofler