It's the job of the sound card to do the decoding, take digitial and trun it into analog sound.
A small audio cabel with a 3.5mm plug goes from the sound card to the speakers.
It's the job of the speakers to amplify the signal from the sound card and make it loud. John
I'm about to get a set of Logitech Z-5500's for my laptop, along with a Creative Audigy 2 ZS PCMCIA sound card. I've read the websites for both, but I really don't know enough about speaker systems to understand what kind of cable I'm going to need to connect everything to the sound card. I'd really appreciate any insight that anyone could give. Also, I'm not sure if better quality is achieved by letting the sound card or the speakers do the decoding.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic