no sound from mac mini via hdmi on an LG tv

I recently bought a new LG 60LA6320 tv with 3 HDMI ports.

I have the DVD player on one, an Xbox on the other and the late 2012 MM on the first. I get sound from the other two, and anything else I've plugged in the HDMI, but not the MM. Before this TV, we had a smaller Samsung TV, and the sound worked fine from everthing, including the MM.

I've pulled everything out, switched everything off at the wall, and reconnected and started everything, repaired permissions etc, and still no sound from the MM. Has anyone got any other tricks? It works back on the Samsung TV, just not this LG one...?

The sound from the TV is going to the DVD theatre system via optical, which I've used for years, and works great with the new LG. Just no sound from the MM.. Selecting the internal speaker works etc, but selecting the HMDI is nothing but .................... (insert tumble weed here)