GE TV loses sound after 1-2 min of plugging to...

I just purchased a used GE 27" Model 27GT720 and have limited sound. I am on cable and get all channels, but lose sound very often except Public TV shows. I unplug and replug to get sound back but it only lasts about 1-2 minutes. Was told owner had it checked at Circuit City 2X and finally called GE 800# and was told to unplug and replug and it began to work. I do the same, but it lasts a short time. What is the problem and can it be fixed? Wanda in TX