I assume that you're using a Tivo box.
Posted by a moderator on the Virgin forum a year ago:
"We're aware that some customers who are using external sound equipment like an amplifiers or surround sound system with their TiVo box connected via an optical connection only, may experience loss of audio on HD channels supporting Dolby Digital 5.1 content. It means the audio from any HD content whether its recorded content, VOD content or watching Live TV will be affected.
Our engineers are currently investigating this issue.
Changing settings on TiVo box to Help & Setting > Settings> Audio> Dolby Digital>Dolby Digital to PCM will restore stereo sound. Alternatively, using the HDMI connection instead of the optical output would restore Dolby Digital"
Source:
http://community.virginmedia.com/t5/Virgin-Media-TiVo-service/No-sound-on-recordings-and-after-pausing-or-rewinding-tv/td-p/2868329/page/5
There are 54 pages on that thread. I've not looked all the way through to see if the Virgin engineers actually found out what was going on but you could try changing the settings on your box from 'Dolby Digital' to 'PCM' or using an HDMI output instead of an optical one.
Of course, if you're not even using an optical connection for the sound output, none of that will apply anyway. (Phone Virgin!).