There are several depending on how the source material is encoded. The simplest (now largely defunct method) encoded the rear channels on the front pair and used a differential amp to "decode" the rear signal into a single mono channel. This is also called Dolby Stereo after its inventor. Modern systems use 5.1 or 7.1 which is encoded digitally and has to be decoded by the same system. Sparkington is correct - DIY is going to be very expensive. A simple (functional card) can be had for a desktop for a few tens of bucks. Soundblaster and others make some excellent units including ones that can utiliise USB so you can fit them to a laptop. With most of these, you'll also need a discrete amplifier and speakers to make it all work. The subwoofer is the most important for most of us - that's what makes the sound sound LOUD! The other channels give our brains directional queues and the speakers don't need to be so good (or large) for gaming.