[tinybilbo]

I have no sync issues with MrMC, Spmc and VLC. Kodi is the only app with this problem on my Fire TV Stick 4K connected to this Sony TV (KDL-50W805B).
I own a Fire TV 2 box. I could check its sync issues with this TV.
I changed the box to the stick because kodi doesn't support automatic refresh rate changes on the box.

Would the results help you? Could take a while to connect the box and update kodi to 18.2.

I checked an old Fire TV 1 box a few days ago with Kodi 17. Same sync issues, different versions of Fire OS. It seems to be a Sony + Amazon + Kodi phenomenon. I'm only reporting it here because it doesn't affect MrMC, so there's definitely a Kodi for Android vector in it. 

It's not just Sony TV's. I have a Skyworth brand Google TV, and I also have audio sync problems with the FireTV.
I don't want to use the onboard KODI/Android (on the TV) because I want to use the UK catchup apps that FireOS has.
KODI, when it's installed directly to the TV is fine (no audio issues), So I would say it's more of a KODI and FireOS/Firestick issue.