First the obvious --
Is it set to mute? Either via the TV, or via the cable box?
Is there anything plugged into the headphone jacks?
Have you inadvertently turned off the internal speakers (on my TV, you can disable the internal speaker via the menu system in case you want to run the sound through your stereo)?
Then troubleshooting --
Do you get sound if you switch to an analogue signal? Or a signal other than the Cable box, eg, a DVD player?
Then, the workaround --
Have you tried turning on sub-titles in the meantime?