I solved the same issue I had since upgrading to Windows 10:
I looked at the advanced properties for the HDMI playback device in Windows 10 and the setting was defaulting to 16 bit DVD quality and on this setting the video on the external screen was choppy and there was no sound. I changed the setting to 24 bit 48000HZ (studio quality) and the sound immediately started to play and there was no longer any lag to the video playback.