Why won't my headphones work with any kind of video streaming website (ie. YouTube, Megavideo)?

Do you have speakers and do they work with these sites? It sounds like the sound source in your audio panel for this audio source is turned down or muted. Check your audio panel and make sure everything is unmuted and the volume is up. If the headphones work with any other source you know they're working and it's something to do with the computer.