Speakers Problem, Help?

I plug in my speakers, and on this stupid-pop-up program called Realtek HD Audio Manager i choose " Line Out" Then I play the "Test" on and it works perfectly, making a sound that would piss anyone of. Then I go to youtube.com ( tried everything, MusicBox, Media Player, iTunes ) and the Sound would not work. WHY? This is a serious problem since i have to play my strat xD. Oh BTW, where do i have to plug in my strat to use it? ( only used Amps uptill now ). Please help, since this is pissing me of sooooooooooooooo bad.