the majority of sounds on windows aren't in surround sound.
this means that youtube, music, windows sounds, etcetera will not be played on your rear speakers.
usually the only things which will produce sound are your left channel, center channel, right channel, and subwoofer.
you can go into your sound settings and set your rear speakers to copy your front speakers so that you get some use out of them but you will have to change everything back to surround sound mode whenever you play a movie or game.
movies and games are pretty much the only things that come to mind which i know do support surround sound on a pc. i have a receiver so i dont control my speakers through the pc but i swap from a mode where my 5.1 speakers act like a 3.1 (rear speakers repeat front) and then i turn a knob to go into 5.1 mode whenever i want true surround (whenever i fullscreen a movie).
 if you're getting noise in the surround sound test then everything is a-okay, its just that your audio source doesnt support surround.