As you may or may not know, the XFCE4 Desktop Environment has a problem with unmute. When you press the mute button on your laptop’s keyboard, the volume goes down, but when you press it again, nothing happenes.
Possible Explenation:
The mute key mutes both alsamixer and pulseaudio, but when pressing the button for unmute, it unmutes only the alsamixer. And you still not have sound.
This is a temporary fix:
$ pactl set-sink-mute 1 0
I have created this alias for helping me unmute the system: alias unmute=’pactl set-sink-mute 1 0′
If this alias does not satisfy you, you can put this command into an executable file, and bind it to a combination of keys.