Is this happening on both TVs, or just one? Is it happening on specific channels, or does it seem to be on all channels? Make sure the coax cable that screws into the cable box is finger-tight, and that all other cables are plugged in all the way. Check your settings as described in http://bit.ly/TV-FixAudioProblems. What kind of cable connects the cable box to the TV? Example: HDMI, component, or RCA cables.
G W J, Thank you for that information. Were you able to ensure all connections to your box and to the TV are tight? Please also power cycle your box and allow 10 minutes for the box to completely reload. Is the sound also missing on SD channels or is it just missing on the HD channels? Thank you.