Your cable is analog stereo cable, it's 3.5mm TRS male to male RCA. You can never get 5.1 on that cable unless the unit require such cable for SPDI/F connection.
You can use SPDIF (coax) connection so you'll only need one RCA cord. Connect it to orange color or black RCA port on either side of the unit. Make sure it said SPDIF or Digital in on the Receiver and SPDIF or DIGITAL OUT on the motherboard. Now set the receiver on Digital SPDIF and your PC Sound Manager output to SPDIF and not 5.1 speakers.