I have tried the sample video playback app on my Samsung Galaxy S6 running Android 5.1.1 and Unity 5.1.2f1 and find that when the AR scene opens the screen displays two triangles that fills the screen the lower left flickering from white, red, pink and the top right black.
When I point at the stones target I see the video placeholder and play button
When I tap the play button the video placeholder turns black and I hear the audio
When I tap the black video placeholder the play button appears on a black background
And all the time this is going on the two triangles remain in the background flickering away?
I ran this test because my app I am developing works perfect on IOS (no surprises there) but I got this result on the Samsung so ran a brand new test project running the Vuforia samples