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.
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.