I just installed my first a770 today. It is connected to a television with hdmi 2.1. Windows detects my tv properly as an audio device, and the hd display driver appears in my devices (twice, just like with the op, apparently). Anyway, no sound comes out of the tv. I tested with a different monitor with speakers, no difference. I uninstalled and reinstalled the ARC driver and disabled and enabled the audio devices and drivers in windows multiple times, and only once, for a brief moment did the sound come on, but it went away after I closed the arc control app and opened it again.