I running into the issue. Got setup Vega 64 -> HDMI -> Sony TV -> SPDIF -> Sonos receiver and I have only Left Audio channel. Windows 10 and Macos exactly the same. Before ATI I had nVidia GTX660 and I had both left and right audio.
Apple TV on same TV run 4K@60 with 5.1 audio.
Where to look to resolve the issue? Hardware problem?