There is a bad interaction between the manner in which Pidgin sets the
sound volume and the flat-volumes "feature" in Pulseaudio, whereby the
master output volume is changed whenever Pidgin adjusts its volume in
preparation for playing alerts sounds. On my system, the result was that
the master volume would increase to 100% (and stay that way) whenever
pidgin played a sound.
One workaround is to prevent Pidgin from trying to change the sound
volume by setting the sound playback method to "Command" and the command
to "paplay %s" in the sounds tab of the Pidgin preferences dialog.
Another fix is to disable flat-volumes in pulseaudio.
See also Debian Bug #541538 against pulseaudio.
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