Last time I used this was at least 5 years ago. I have never had an actual soundcard since like... 1998. Always onboard.
I am pretty sure my old, old computer only had 3 and all the speakers worked?
Unless you had an adapter of some kind, I'm not sure how. The green plug only feeds the front two speakers, and possibly the sub (although most of the time the sub and centre channel share the same jack).