Make the Connections
These primary connections might work for you, depending on the equipment you've got on both ends of the connection:
Old-fashioned RCA analog audio jacks on your sound system (linked to the audio out from your sound card). This requires a special cable with 1/8-inch phono jacks on the sound card end and 1/4-inch RCA connectors on the sound system end (or an adapter on a typical high-quality RCA-to-RCA cable, which is what I use for stereo sound). Except for multichannel analog links (available on break-out boxes or add-on connector panels from high-end sound cards such as the Audigy 2 ZS Platinum Pro and Audigy 2 ZS Platinum cards, respectively), this tends to limit sound to two channels.
Optical connectors. These connectors are making the scene on new sound cards (or their daughtercards and break-out boxes, more typically) and on newer audio gear. If you can make the optical connection between both sides of the link, you'll be able to move huge amounts of data and get some of the best sound quality aroundprovided, of course, that your sound system can decode and deliver the multichannel audio scheme that your sound card produces to its speakers.
S-Video connectors. Although these connectors were designed primarily for audio and video, they provide a clean, high-bandwidth mechanism to deliver multichannel sound from your sound card to your sound system (subject to the same caveats as for optical cables, of course). That is, you must be able to make the connection, and the sound system must be able to decode and deliver the multichannel audio information to your speakers.
Coaxial cable connectors. Some sound cards also offer coaxial cable connectors such as those used for CATV connections, and some high-end audio equipment does likewise. Personally, I've been unable to test the workability of this kind of connection, but if you've got what it takes on both sides of the link, it may be worth experimenting with. Here again, conventional CATV coax can carry plenty of bandwidthmore than enough for eight or more channels of 24-bit audio, in fact.
Of these choices, the optical or coaxial cable connection is the most likely to require new equipment investments, often on both sides of the link. High-end receivers and pre-amps have offered one or more S-Video inputs for years now, so this might be a good avenue to try, as long as your sound card offers S-Video output and you have an extra S-Video input to spare. But if all else fails, you can always get pretty good results from line out on the sound card to an input on your sound system (and then even synthesize decent multichannel sound from such input, depending on what your sound system can dofor example, my old Sony pre-amp does a pretty creditable job of converting such input into five-channel sound and can even produce 10 different preset soundscapes in which to play the resulting output).