I've just hooked 2 echo studios and echo sub to a Fire TV cube v2. Everything works fine and is lip-synced (after adjusting the lip sync settings on the Fire TV cube) ... except the YouTube app (Google Play purchases and normal YouTube videos alike).
Either...
(1) The video starts to play but displays a black screen with only half a second of audio and then stops. The round marker on the progress bar keeps moving back and forth slightly, like it's stuck in a loop.
(2) The video starts but audio is way out - more than a second behind the video.
(3) Same as (1) but pressing play starts the video again and the same audio sync issues occur as in (2).