8-channel refers to how many different.... channels... your sound chip can split sound into. Your motherboard allows you to have 8 speakers set up, and each one will function on a separate channel, which means making their own sound appropriate to the music/game/sound effect etc.
Your motherboard also comes with software for controlling the sound chip. In this software you can configure it to work with varios setups, including 5.1 speakers. Just mess around with your settings, and you'll find it in there somewhere.
If you can't find it, tell us what your motherboard is, and someone can help you make the adjustments.