Oh, you're not thick - it may not be your problem at all, it's just my neighbour recently had a problem with his sound after buying a new tv & it was resolved by programming his UPC remote (just had to enter the correct code for his make of tv) to adjust the volume on his tv. There is a list of codes for different tv's on the UPC site.
I don't want to set you on the wrong path, so it may be best to ring UPC or see if you get any more helpful responses here.