Speakers aren't working

I just got a new computer with Vistsa Home Premium. I have my old Harmon
Kardon subwoofer and two attached speakers hooked up to this new computer but
I get no sound. My "volume-panel" display shows/detects only my
frontcenter/subwoofer jack, perhaps because my left/right speakers actually
jack into the old subwoofer which then jacks into the new computer. When I
sound test, I get a little fuzzy subwoofer static and a voice comes through
the speaker on the new computer case speaker, but I get no sound when I click
around to the other speaker icons. Any ideas? Thanks in advance for any
help.
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rich60