Not meaning to be disrespectful but have you noticed problems with your hearing with other things such as using the telephone, speaking to friends or such like?
You may need to adjust the 'treble' setting and turn down the bass.
How old is the tv, is it Nicam ? Maybe the signal is being interupted and is trying to switch between Nicam and mono and so losing sound that way.
There are hundreds of things it could be but probably only one that it is.
It may just be the tv has a fault, best to get someone round to listen to it and see if they can hear it going funny.