Try isolating the problem.
The first thing I would try is running the tv directly from the cable box. Assuming the HDMI cable is good, this should give you an incredible picture and whatever sounfd the tv is capable of.
Once this is done, then try the other HDMI cable. This should produce the same result and now you'll know te TV and both cables work.
Now, the only variable would be the receiver. I'd check the connections and setup very carefully (these are NOT intuitive). but the possibility of a handshaking issue between the STB and the receiver is always a possibility.
If you had an upconverting DVD (or a blu-ray) player you could test with another source.