Not a dumb question at all, but perhaps better asked prior to purchase.
I'm not sure what your best option is, but here are a few:
1) Stick with computer speakers, but return your Swans. They are a 2.0 system. Purchase a 2.1 system, which will include a sub.
2) Move up to home audio type speakers. Return the Swans and purchase an audio amp, un-amplified speakers, and one of the subs mentioned above.
3) You might be able to split the signal from your sound card. Keep the Swans, buy one of the subs above, get the parts to split the signal & see if it works. If it doesn't, it might work if you add a headphone amp between the sound card and the splitter.