What speakers are you using? ie. stereo/2.1/5.1 etc.
It sounds like you have more channels enabled than you have speakers!
If you don't already have it installed, go to the Asus Support site for your board
and download the audio driver
and utilities. The board has auto jack sensing, so you may be able to just unplug your speakers and then plug them back in again to have the board auto adjust the sound settings, if not, load the sound manage application, which I think will be Soundmax, and adjust the audio output to match the speaker configuration that you have