Why isn't my computer making sound?

I have speakers that have a green light on them meaning they're on, but when I go on a website like Youtube that needs noise, no sound comes through the speakers. There isn't a speaker button on the bottom right side of my computer (Windows XP) either. When I go to my volume control it says there isn't any speakers connected to the computer and there is! I need help!!!!!!!!!!!!!!!!!!!!!!!!!!!!+