Go to start>settings>control panel>sounds and audio devices. On the volume tab check if the mute was checked. Go also to advance tab to check if all is set to a volume level. But if the dialog box appear in grey color, you need to install your sounds driver.
I already used the Help and Support Center, it said: Go Start > All Programs > Accessories > Entertainment > Volume Control, but i don't see the volume control there. Then i went to Control Panel > Add/ Remove programs > Add/Remove Windows Components to install > i can't find it too!