help choosing cable pc to dac.

I use a USB?SPDIF converter with older DACs. The V-link, as suggested by markomaras, can be had inexpensively.  What will you be streaming in way of file type?

I take it that your sound card has the 3.5mm output. I see no reason a 3.5mm to dual RCA cable would not work for you. 

The V-link II for sale is mine. It only goes to 24/92. Though I agree that a USB/SPDIF converter would serve better, it might be that a entry level cable will serve the OPs' objectives regarding background music listening. I bought a 20' cable on E-bay that would suffice for ~$25. 

As a follow up on my last post. The OPs' sound card outputs through 3.5mm analog using internal DAC or SPDIF using optical. 

A USB/SPDIF converter will not serve unless the OP chose to bypass the sound card and use USB which would require the purchase of a USB DAC or USB/SPDIF converter/DAC combination.

Regarding use of the 3.5mm analog output the choices are: 1) the 3.5mm to dual RCA cable under consideration, or 2) the suggestion made by audioman2015, use a 3.5mm to dual RCA converter with standard RCA interconnects.

One other option would be to purchase one of the inexpensive DACs from China off Ebay with optical input along with a toslink cable and a pair of RCA interconnects. My question would then be how to best divide the 18' between the two cables.

Since your interest is for background music I suggest you try it.  

Somehow I seem to have let myself get confused. After rereading this thread I am now aware that you have a soundcard with coax output and a DAC with coax input. I would use a 1.5M coax between soundcard and DAC. Use a Pair of RCA interconnects of the desired distance between DAC and preamp input.