The problem is most likely your SAP being turned On.
SAP was created for audio programing including translations into a 2nd language. On nearly all sets, the SAP being ON will essentially select an alternate audio stream and will de-select or mute the main audio from the channel your are viewing. However not all TV stations have SAP enabled.
I am the Chief Engineer of a TV station in Denver, CO. We are not using SAP so if you selected SAP while watching my station you would get noise or static. There would not be any audio there.
Go to the front panel of your TV and select the "MENU" function. Step through the menus to Audio features and De-select SAP or Enable your main audio and this should take care of the problem. Alternatively you might be able to unplug your set for a period of time and it would reset itself. I used to get calls from elderly viewers when the set wouldn't pick up the UHF TV stations anymore. These were always after a power failure and the set had defualted to factory settings,
Different set manufacturers use different menu schemes and withou a password feature, these menus are so simple a small child playing with those fascinating buttons can change them for you to something you really don't want.
Good luck and write back if you can't find it. The set manufacturer and model number would help me look up a menu tree for your TV set.