This tv needs a minor adjustment to the sound if detector coil. I am attaching a file I got off the internet, its a schematic for some admiral tv. You will need to make the adjustment called "4.5MC Sound IF Alignment". I can't find the one for your tv, but you should get your schematic off ebay, should be around $12. It will have an alignment similar to the one in this file, but will have the location of the coils on your tv.
This is an adjustment you can make with your signal source, just set the tv for a clear sharp picture, then adjust the sound coil. follow the directions, warm the set up.
You can make this adjustment as long as there are no other problems with the set. No drifting of anything over time, sound does not get better, or worse if left on for an hour.
Also you will need the plastic alignment tools, they are about $12. on ebay.
http://www.ebay.com/itm/GC-Radio-TV-...vip=true&rt=nc
Be sure you make the adjustment carefully, the coils are delicate, most likely brittle too. Turn them as directed, but if there is no change put them back to the spot you started from, and be sure you have the right one....
Don't turn random coils if it does not improve.
Why did this happen....? Some component most likely is changing value, if its going to keep changing you will have to find it and fix it... there is a chance, however that it will not change again any time soon, so this will fix it for years to come.....
Good luck....