I encountered the same thing using the HDMI interface on my new TWC cable box (Motorola DCX-3200). After a little bit of research, I finally fixed it by changing the digital audio
mode in the cable box using this procedure:
1. Be sure the TV is on and connected to the cable box.
2. Using the cable box remote turn power OFF.
3. Immediately press the MENU key on the remote. The USER SETTINGS menu should appear on the TV screen.
4. Select the ADDITIONAL HDMI SETTINGS >>> menu item.
5. The ADDITIONAL HDMI SETTINGS menu will appear.
6. Scroll to the AUDIO OUTPUT item and select this.
7. Change the setting from AUTO to L-PCM.
8. Return to the main page
9. Using the remote, cycle power OFF.
10. When the cable box is turned on again, you should be good to go.
I realize these instructions are specific to the DCX-3200, but there may be a similar setup option available in your cable box as well.
The problem here is that the cable box is by default configured to use Dolby Digital audio, which requires an additional 10dB of headroom to deal with the boosted LFE channel. If the cable operator hasn't taken pains to set the system up correctly (and most haven't, apparently), the digital audio level sent on the HDMI interface will be approximately 11dB down from normal analog line levels on the HDTV
channels. Changing to straight linear PCM sets the TV receiver audio level on the HDTV channels to match the levels on the non-HD channels.
The only problem with this fix is that if you intend to feed a Home
Theater receiver that uses Dolby Digital processing with the cable box output you will lose the Dolby encoding feature and be forced into straight 2-channel stereo. For a simple installation like mine where I'm just feeding the TV, this fix works fine. If you have a more complex setup that takes advantage of Dolby Digital features (when it's available in program material), you may have to live with the problem.
For a technical reference on why this happens, see http://www.megahz.com/specimages/Dolby/LM100%20Cable%20App%20Note%2011-05.pdf
Hope this is useful.