Sony support confirms both ends need to be eARC. An option is optical, but that may not provide better sound than the surprisingly good sound with the Sony speakers, although I think it could provide surround, I'll give it a try.
As an aside, I was a bit disappointed the beautiful 85" video didn't quite have the impact of the 100" 1080p projection it replaced. I'll keep my Stewart screen. Maybe someday the price of 4K projectors will come down.