I have Comcast. I have a TV in the basement that I rarely use - so I don't want to rent another box from Comcast. When I plug the coax TV cable into my TV, I get about a dozen stations (ABC, NBC, FOX, etc). I'm okay with that, but I'd like to be able to use my surround sound setup for the sound.
Do you know if the tv is picking up Comcast's analog channels or Clear-Qam ones? Ie, are you switching to channels 3,4 or 5 or 17.1 23.1 etc?
If the former? Is the TV HD compatible? If not that an SD Converter box will probably be good enough, but I don't know if one exists with digital out? You can probably get sterero out of it happily and may even get something emulated with ProLogic. This box would work for that scenario: http://www.amazon.com/gp/product/B003OQGJV6/ref=oh_details_o01_s00_i00
However, if you're wanting HD, the options get more exciting. The first thing to check would be if the TV itself had a digital audio output and try that. The digital tuner is in the TV, so that's where the audio is decoded. There are HD Clear qam boxes out there, but the early ones are hard to use and are expensive.
Not quite sure what you mean by "working", see if it has a digital out and try it anway, on mine it's a coax, so a simple rca lead was good enough. The sound is output before the audio decoder of the tv works on it, so there's a good change it will work even if the speakers are hanging off having been eaten by your dog.. (Told you I didn't know what you meant by "working"..).
If not, I picked up an HD box made by Samsung. It's fairly horrid though as it won't use the psip channel data, so if the channel is on cable channel 65, but psip says it's 17.1, it'll come over as 65-1.
Also, there was no way to get . or -, so you had to land on the first channel and channel up through it's subchannels.