You should have some kind of output on the TV, take a look and see what you have.

If nothing else you could use the headphone output but if there are better options use one of them.

If the cable box presently connects to the TV over HDMI then look for an HDMI audio extractor, with built in outputs to go to your TV HDMI Input (pass-through) and with RCA audio out to goto the Connect-Amp line-in. Perhaps search Amazon, as there are usually plenty of such devices on that shopping site to view/research, before buying.

edit: Something similar to this - just an example I stumbled upon (it’s not a recommendation) - and suggest you research further, prior to any purchase…

https://www.amazon.com/LinkS-Extractor-Splitter-Converter-Chromecast/dp/B00XJITK7E

What do you want to do with your Connect:amp that needs a connection to the settopbox? Using it for TV sound results in a 75ms delay in sound that would make this solution unusable for me. If you only need it for radio (where the delay would not matter), Sonos can play those when you choose them through the app. 

Thanks a lot for the replies.  I am going to try the HDMI audio extractor and see if that works.  I could definitely not deal with the 75ms delay!  I will let you know how it goes.  Thanks again!