Why wont my speakers work on my computer?

I have logitech speakers, they were working ok accept one, and my friend was getting rid of some of his speakers so i switched his ones for mine, which I decided the sound wasn't clear enough, so I switched it back around to my logitech ones, they now don't work. What I mean by don't work is that it will only work when I try test them using the on screen sound tester, but when ever I try play a file that has sound, whether its you tube or I-tunes, it doesn't play any sound. I have checked the mixer and it is all checked to be OK, why is this? (I have windows vista if that helps.)