RF Modulator
Wrong, it's a RF modulater, not a demodulater which is something entirely different for AC-3 audio.. and a RF modulater converts a composite video and audio signal to an RF signal, NOT the other way around. So not only was your advice bad, it was inaccurate as well. This will not work for him, and neither will that stupid RCA to Coax converter! If you guys don't know what the F*** you are talking about, stop giving advice and stop jerking this guy around.
If your cable box has rca video out, it will be labled either video, or be yellow in color, use this to your receiver for video, and the red and white for audio.
If it only has a coax connection, connect this to the coax input of your vcr, then connect the composite outs from your vcr to your reciever. You'll have to switch between TV/VCR on your vcr to see cable, but this keeps you from having to switch inputs on your TV. Basically the receiver input will be the same for both your vcr and cable box, switching between each with the Tv/vcr button on your vcr remote. It may not be the ideal thing for you but it is what will work best with what you have.