I have a Nexus 5 with Android 6.0.1.
The movies will go dark after 20-30 seconds if you don't touch the screen to keep it awake. The sound will return in about 5 seconds and keep going with the screen remaining off.
It seems to happen with all movies, live and archive. I am watching Spartacus live at this moment. A minute ago I tried the archived Battleground. All with the same result.