The motherboard has optical out which I assume you know. I just mentioned that for people who may read this post. The mobo does support Dolby but is optional and what that means unless the mobo manufacturer pays for the licenses(which would make the mobo more expensive), it doesn't have the software. For example here are the optional items.
Optional: Dolby PCEE program, SRS TruSurround HD, SRS Premium Sound, Fortemedia SAM, Creative Host Audio, Synopsys Sonic Focus, DTS Surround Sensation | UltraPC, and DTS Connect licenses. and link:http://www.realtek.com.tw/products/productsView.aspx?Langid=1&PFid=28&Level=5&Conn=4&ProdID=284
I'm also guessing that you don't have the Astro mixamp which has Dolby digital. So you would need to buy the mixamp, or a soundcard or pay for the Dolby digital software.
Another thing I would like to mention, SPDIF(optical/digital) only supports 2.1 uncompressed audio so anything above 2.1 will be compressed. Analog, HDMI and display port do support true, discreet and uncompressed audio for 5.1 and above. What this means with SPDIF and the Astros, you can only get simulated 5.1 and above.
I hope this clears some things up but if you have any other questions please post back.
Be seeing you, the Prisoner...