Sorry, I assumed you were using onboard audio, since no dedicated soundcard was mentioned. Things are looking up...I do have a seperate card and I use the digital audio out (I don't use the input at all) to feed the surround to my surround AV receiver and use that DAC (Digital to Analog Converter). You mentioned 'output' but actually you need to feed the OUTPUT signal from your Astro into the INPUT of a DAC. I did assume you meant INPUT anyway, but we need to be sure. The DAC can be in different forms, from an AV receiver, to the built-in DAC on a sound card, with either HDMI, digital COAX or optical TOSLINK inputs.
You can get a soundcard with SPDIF (TOSLINK) INPUT, such as this one...