結論
sudo nano /usr/share/pulseaudio/alsa-mixer/paths/analog-output.conf.common
でファイル内容を以下のように変更する。
before
[Element PCM]
switch = mute
volume = merge
override-map.1 = all
override-map.2 = all-left,all-right
after
[Element Master]
switch = mute
volume = ignore
[Element PCM]
switch = mute
volume = merge
override-map.1 = all
override-map.2 = all-left,all-right
[Element LFE]
switch = mute
volume = ignore
参考URL