Hi, and welcome to the forum!
You can either connect the audio output from your TV to your Onkyo, or you can connect an audio output from your Comcast box to your Onkyo. Either should work, and it doesn't need to be HDMI.
I imagine that your TV has an optical audio output. Do you know if it has ARC (audio return channel) on the HDMI connection? If not, you can let us know the model number and we can look it up.
Also, which audio outputs are available on your Comcast box?
That said, I agree that it would be best to get an HD cable box with your TV. Would that cost you extra?