Poor sound quality since moving to Virgin

All channels through the Virgin stream box have poor sound quality, muffled speech especially bad. Some channels are worse than others, but viewing dramas and films is particularly bad, resorting to putting on subtitles.

When viewing via other platforms, apple, prime, Netflix etc directly through the TV app there is no problem. I've tried changing the options in settings but this makes no difference. This is annoying as I'm now locked into a 2 year contract. Please advise