It's quite common that when I turn the TV on to watch it, the sound will not be working.
I have to, in the end, completely restart the box. It is very annoying. Turning it on standby, changing HDMI or port does not do anything.
I've had this issue since week 1 with Virgin.
Go into audio settings and enable Dolby Digital to PCM which will convert to stereo, helpful if your TV is struggling with audio conversion.