Recently I bought the Xiaomi Mi TV box 3 (International version). From the specifications and description, it seemed perfect for me as it is cheap and supports Netflix 4K, Chromecast, Kodi and running Android TV which gives access to many useful apps from PlayStore.
To cut long story short, from day one I noticed problems with video and audio being out of sync in video streaming apps like Netflix and YouTube. I searched through forums to find a solution but nobody, not even Xiaomi official responses have given a proper solution.
For all you frustrated folks out there, this is the solution you have been waiting for:
Settings -> About -> tap on Build until developer options enabled.
Then go to Settings -> Developer options -> General -> HDMI optimization and set it to Best frame rate.
Restart the device and hopefully everything will be working as expected.