I am not sure about your soundcard's output, but if it's a stereo out (accepts 1/8 " TRS) then you'd need a Y cable with an 1/8th inch TRS plug on one end and a pair of RCAs on the other, then put RCA to 1/4 inch adapters on that.
I have not seen a Y cable that goes directly from 1/8 inch to 1/4 inch, but they may exist in this cable-crazy world we live in.