Answer 1: From the terminal, run
xfce4-mixer or even easier, if you have the speaker icon on your panel (i.e. which you can easily add to your panel if it's not there, just right-click on the panel and move around, it's not hard) right-click it, chose to run the audio mixer, make sure you are on the playback tab. Your master, headphone and speaker levels should be un-muted and high enough for you to listen to anything.
The problem is that when you mute your volume again, only the master responds reasonably. But the speaker and headphone levels get muted and never back to un-muted. So I have to manually un-mute the headphone and speaker levels.
I tried a solution which didn't work for me, but may be for others. It's about sudo editing a file: /etc/modprobe.d/alsa-base.conf and append this line to it: options snd-hda-intel model=laptop-eapd, then reboot, but this didn't work for me.
I'm using a Dell Inspiron SE 7520, Ubuntu 13.10, Xfce 4.12.
Answer 2: Log out of Xfce, log in using Unity, un-mute your voice (This is the trick. I found my sound muted on Unity). Logout and login again using Xfce.
What still bothers me, is that when I muted the sound on Xfce, the problem occurred again. No sound at all even if I un-mute it while I'm still on Xfce !