I just recently recieved a logitech z560 from a friend. I only have 2 of the 4 speakers and i dont mind but when i set the system up and tried to plug it into my computer i got no sound out of it at all and anything i look up on the internet keeps saying something about a soundcard? i dont know. and the system worked fine when it was at my friend's house before she gave it to me. does anyone know what could be wrong, or what i'm doing wrong? any help would be greatly appreciated!