I have an Ultra in the living room (connected via ethernet) and 3 of the Streaming Stick+ (4 TVs total)
I've had this audio sync issue on all of them.
I first notice it as "lip sync" being off a little. It seems to progressively get worse if I do nothing.
What I have found that fixes it *almost* all the time, is to hit Options (while any video is playing) and turn off Sound Leveling. The audio syncs. Then I turn it back on (I like it!)
My understanding is that the OS 9.1 update addresses this specific issue. I have not yet received the update on ANY of my Roku devices to confirm or despute that notion.
One of the bedroom TVs the sound got so bad it would stop and stutter. the sound leveling "fix" didn't fix that. I had to restart the streaming stick+ and then the problem was resolved. HOWEVER... I think that may have been a network issue. I'm still getting a handle on which rooms can use 5GHz wifi vs the rooms that do better on 2.4GHz.
My ROKU Ultra has horrible voice sync issues when using Youtube. I am just going to leave Sound Leveling off till 9.1 gets here.