Picture but no sound when recording from TV

I have a Sanyo VCR (model VHR-874E)connected via a SKY digi box.Up to a few weeks ago I had no problems with recording. Now, though, when I try to record from terrestrial TV and play it back I have picture but no sound. When I record channels from the SKY box ( including BBC etc) everything is fine. Old recordings playback with picture and sound. I've tried everything: disconnecting and reconnecting, retuning the VCR and TV channels but to no avail. I would be grateful for any advice