Sound doesn't work on a Centos 6.5 VPS

I installed GNOME Desktop Environment on Centos 6 on my VPS, then I installed Firefox, Flashplayer and VLC. Everything work well but I can't hear the sound. I searched for a solution on the Internet but couldn't find.

Here are 3 screenshots for the system, the sound output and the sound card after running 'alsamixer' from the terminal.