19dv667db lcd ?

I recently bought this t.v off a friend who was buying a bigger T.V. This t.v worked fine for 2 months after purchase but I am now having problems with the sound. When I switch the t.v. on It works fine but as soon as I turn the channel over the sound goes off. I have tried the dvd and the same thing happens. When you turn the volume up you can hear noise and voices but they are muffled and the voices are like some one is talking like a dalek from Dr Who. This happens in both t.v. and video mode. I have checked the aerial and there is no evidence of it being loose. I would be grateful if you could inform me what is wrong so I can get it fixed. I do not have the original receipt so I can't even take to the shop my friend bought the t.v. from.