Oh, I see. I thought you meant you only had sound missing on HD channels.
DVI only supports video, not audio, so you need another way to get the sound to the TV. Your TV should have a separate audio input that's used together with DVI - probably either phonos or a stereo jack socket. Check the TV manual.
Then you need to buy a new lead to take the audio from the VCR SCART socket on the V+, to the TV. Usually one of these two will do the trick:
http://www.maplin.co.uk/scart-to-3-p...t-switch-28102
http://www.maplin.co.uk/scart-to-3.5...t-switch-43876
On either of these cables, leave the yellow plug dangling. It carries composite video, which you don't need.