Think I've figured it out - it will play sound when I select interlaced resolutions (1920x1080i, 480i etc) but not when I use standard progressive scan ones (1920x1080p).
I was going to get a better TV anyway but thanks for all your help
You've been reduced HDMI data rate by doing so. Chances are your HDMI cable can't handle higher data rates, so it drops the sound. Get an HDMI 2.0 or 2.1 cable before getting a new TV.