Thank you for your question. What are your sources? Do you have a cable or satellite tv box? Typically if you have a cable box, you would come out of the co-ax (SPDIF) output and directly into the co-ax digital input on your RCA system. Then you would plug an analog (RCA) cable from the cable box audio output into your tv if you wish to sometimes use the speakers built into your tv. Lastly, you would connect the video of your cable box to your tv. This is typically done with either component video cables, or HDMI if your tv and cable box are so equipped.
The reason you did not get sound out of the digital co-ax output on the back of your tv is that it is not active unless you are using the HDMI input. In this particular instance if your cable box has an HDMI output, you can plug directly into the tv and then use the digital co-ax output to get the sound to your RCA stereo system.
I hope this helps, if you need further assistance please give us a call at 888-880-8992
Thanks,
Aperion Audio