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.
Your server only has a “Dummy Output” for sound, so it's normal that you don't hear anything. (That's dummy in the sense of “a deliberately nonfunctional device or tool used in place of a functional one”, it's a coincidence that it happens to match the “mute” meaning of dummy.)
If this was a physical server rather than a VPS, you would be attempting to play a sound through the server's loudspeakers (you wouldn't normally have the permission to do so when you aren't using the console though). VNC doesn't forward sound, only keyboard/mouse input and windowed display.
You'll need to set up sound forwarding separately. See
