That is the issue, you cannot mix digital (hdmi) and analog (rca) connections, so you wont be able to get sound from cox through the tv.
What you could do is run a set of red/white from the cox box to the amp, but that still wouldn't give you sound from the smart functions on the tv like netflix.
For that you need an optical output, but the amp is too old to even accept that.
Updating to a new receiver is probably your best long term option.
The way around this is to use the Digital Audio Out port on the back of the tv. This is a square type port with a little flap/door over it and a red light inside. You connect a fiber optical cable to it. You'll need to purchase a digital to analog converter: http://amzn.to/1VhPiQx . This will convert the optical connection to an analog stereo connection (red/white rca ports). To that, you can then connect rca cables to the amp.
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