I realise that this will probably be incredibly esoteric, but if anything a potential issue is brought to someone's attention.
I've recently gotten a 5.1 setup which uses optical connections. This has, from the get go, been fine. However, recently, the V6 box has been having issues with it. Watching Amazon Prime last night, the program had 5.1 audio, there were several audio dropouts, lasting 1 - 2 seconds. It also did this a few days ago as well. And eventually it started to sound like the audio was coming through in slow motion. This forced us to switch to the xbox one, and carry on there. No problems, at all. However, the sound through the TV (hdmi cable) is fine.
I have discovered that my TV has an optical output, so assuming that the 5.1 signal is forwarded over hdmi and passed through to the optical output, this would be a more convenient system, since only 1 cable needs to be used and no faffing about switching between devices like I have been. Hoping it solves the issue, at least with Virgin.