Throughout this discussion I didn't see you mention the brand of cable box you have. This might be helpful in your configuration process. I have one(Scientific Atlanta) that will output both HDMI video AND coax cable audio at the same time. In my case the HDMI "video" connection overrides all "video" in my set-up so even if I go in to the menu and change the audio to DD or "digital" from HDMI, the HDMI video connection will remain as is. I do have it hooked through a relatively new receiver which takes HDMI but generally speaking the cable boxes are quite behind the curve in being up-to-date on their connections(Usually only 1.0 HDMI compatibility). This configuration will ONLY allow 2channel audio through HDMI. I still have to hook coax through my receiver, much like you do because the HDMI audio from the cable box cuts in and out and is unreliable anyway, hence the dual connections. DD from television stations can come in many forms from 2 channel up to 5.1 and anywhere in between. Provided we can solve this audio cable box problem, you are still better off trying to hookup the audio from your cable box to the receiver as opposed to the optical out of the monitor.
I am sure there is something in the set-up menu that will allow what I described. The fact that you have the HDMI video connection going directly to the monitor shouldn't make any difference.
Let us know the brand and any manual you may have that can be referred to so we can then take it from there.
