ASUS is easy to get to, and appears to have fairly good descriptions of their current boards. It also looks like they have support site, and a "USAS Club" which I would assume is a user's group where you can likely get answers from people familiar with your specific board.
The couple of USAS boards I took a quick look at all claim to have "6-channel" digital sound. If that's what you've got, the connections (jacks) you're looking at are probably not for left and right channels as you might expect. The PC powered speakers most commonly used/supplied with recent PCs I've seen plug left and right channels in on one "stereo" connector, usually using a plug connected to the right speaker, and the left main speaker plugs into the right speaker. Any other jacks are often for the "surround sound" auxiliary speakers.
The USAS motherboard (US) page I looked at indicated some downloads available, and it is possible that you need to download and install drivers and/or control programs for your sound functions.