Main Suggestion (1)
If someone has previously connected headphones directly to the TV then it is possible that the mechanism has not sprung back into position to resend the sound though its speakers. Try inserting and removing the headphone jack a few times to see if will loosen it enough.
(This has happened to me on a Sony Trinitron jack socket.)
If you are certain that nothing has been inserted into the jack socket to cause this then do not try to fix this way as it may add an additional reason for there being no sound!
Suggestion (2):
Slim chance on this suggestion. You helpfully state that you have tried another cable box. Make sure they were both connected up OK. Have you also a DVD player or something else you could try instead? (Tedious I know.)
Suggestion (3):
Find out what has been done to the TV/Cable-box/cables in-between the sound last being heard and the sound first being missing. Hopefully that may give a further clue if it was manual intervention that triggered the fault.
NS, October 2012
