3 gsettings
set org.gnome.settings-daemon.plugins.media-keys volume-mute
'XF86AudioMute'
4 gsettings
set org.gnome.settings-daemon.plugins.media-keys volume-down
'XF86AudioLowerVolume'
5 gsettings
set org.gnome.settings-daemon.plugins.media-keys volume-up
'XF86AudioRaiseVolume'