Comment 2 for bug 883485

The problem is that xfce volume control unmutes alsa master, but not the pulseaudio master.
I guess it's due to the wrong default settings of alsa or the xfce volume control. A workaround that didn't involve running scripts and worked for me was to
1. create /etc/asound.conf with

pcm.pulse {
    type pulse
}
ctl.pulse {
    type pulse
}
pcm.!default {
    type pulse
}
ctl.!default {
    type pulse
}

And then to add a keyboard shortcut (via settings -> keyboard)
XF86AudioMute -> amixer set Master toggle