Update - a short while after posting this I went into the Roku menu to set the auto detect back on (under HDMI in AUDIO in SETTINGS) , because I had taken it off auto-detect while trying to fix the issue, and now the issue has resolved - unless it was a coincidence and something happened elsewhere at the same time -- that has FIXED THE ISSUE.
There is no logic to this fix because we only took it off auto-detect today while trying to fix it. But perhaps taking it off auto and putting it back on is the fix.
So what seems to have worked is going to SETTINGS and then AUDIO and then HDMI and then taking it off AUTO DETECT and then going out of the menu (and maybe turning off the whole thing at the mains and back on again) and then going back in and putting it back onto AUTO DETECT again.
Roku streaming stick + 3810rw connected via HDMI powered by its own power supply. I have been experiencing the same sync issues mostly on Hulu. None of the above fixes have worked. What does work but is annoying to do is after the video starts either rewind or fast forward and the sync issue disappears.