Unfortunately, you are just finding out that these "All in one box" solutions limit your options tremendously. Sad to say, my answers don't offer much solace...
Not that I'm aware of but, odds are that the HDMI input on your panasonic is for video only. For HDMI to be able to handle audio, it needs to be version 1.3 and this is just now starting to trickle out on high end gear.
This you need to take up with your cable company but be aware that cable companies are notorious for having problems with HDMI sound issues. Most rely on Toslink or digital coax for their 5.1 audio.
And, not all HD cable boxes evem have HDMI video output. Many still rely on component and DVI. Again, check with your service provider.