[tinybilbo]

I've got the same problem with my firestick 4k (gen3). You'll see that's it a problem for others as well over on the Firestick XDA forums as well.
I can't find the actual posts as it was (if I remember correctly) just a couple of answers within various posts..

Turn off the Dolby setting in sound preferences and it should be fixed.

Not an ideal answer...

*Edit*
It might not have XDA, here's a couple of other links where it's mentioned.

https://www.reddit.com/r/fireTV/comments...ync_delay/

https://www.amazonforum.com/forums/devic...-audio-lag