I'm joining this thread so I can track the progress of the fix as I am also experiencing this issue. With surround sound turned on on the Chromecast I had unfixable audio delay with both the Harmon Kardon Citation Multibeam 700 and more recently the Sonos Arc. The TV is a Samsung 65QN85A and the Chromecast is plugged into the TV. I noticed little change in the delay when connecting the sound bar via optical audio or HDMI eARC but the problem is immediately solved when I disable the surround sound option in the Chromecast settings.