I have one of these through TIME WARNER. Turn the cable box off (soft off - it needs to be plugged in still).
Hit MENU (either on your remote or your cable box). What should come up is a menu (white screen with black text). From there you can set the aspect ratio for your cable, the resolution for HD content (1080i, 720p or 480p), and the resolution for standard 4:3 content (480i, 480p or "stretch").
There should be an option for ADDITIONAL HDMI settings. Go there and click ENTER. On the next screen, you'll see an option for the video (HDMI or DVI), the color space, and the audio. I have mine set to AUTO. Whenever I change to PASS THROUGH, I get no audio through my HDMI. See if yours is set to AUTO. I'm thinking that should take care of your audio. I believe if it's set to PASS THROUGH, it only sends audio out through composite (red/white).