So you TV and the receiver came without a manual? At least for the receiver you can download it here: http://www.eu.onkyo.com/en/products/tx-sr313-75901.html?tab=Support
It will tell you how to connect the TV and why there is a matching orange port at the receiver.
Thank for the response. Yes, I've downloaded the .pdf manual and read through it. I've gone through all the settings and tried almost every combination of settings on the receiver AND the audio setup on the tv itself. I just can't get sound to output to receiver speakers when watching cable. I guess I'll go the electronics store and buy a digital coax cable and see if that is the issue.