You should only need the HMDI cable connected between the TV and DVD/HD recorder. I have the Sony RDR-HXD770 connected to a Samsung LCD TV via HMDI with no problems.
You could try removing the HDMI lead and reseating it.
Try the HDMI lead in another HDMI socket
Or try another cable.
Just a thought if it is the Virgin channels that you are missing the sound from and this is connected to the DVD/HD recorder via scart lead it could be the scart lead that requires reseating.