Tried that....doesn't seem to make a difference. And holy cow the issue is even worse now. I basically cannot watch the series I'm trying to watch(Babylon 5). I have to play it using the built in Amazon App on my TV.
I have tried every HDMI Audio setting to try and get it working. Netflix seems to work fine. Watching Youtube videos are fine as well. I can jump around on Amazon Prime and get the sound/sync issue to duplicate but it doesn't occur on every video.
I have a newer LG 43 inch 4K tv in a bedroom using a Roku Streaming Stick +. It seems to work fine when trying to watch the same series on Amazon Prime.
At this point I can only surmise there is some kind of compatibility issue with the Roku Ultra and Amazon Prime. Or some other incompatibility like HDMI, etc