1. If your motherboard have HDMI, use HDMI to output the audio signal.
2. Check your motherboard, if the coaxial output have coupling transformer, then use Coaxial.
If your motherboard don't have output transformer for coaxial output, I suggest you using Optical output.
3. If your budget is allow, try to purchase AMD (ATI) HD 5570($610), or HD5550 ($4xx). It have Dolby True HD and DTS-MA output on HDMI port.