While you are watching the video, make sure you're not holding on forward or backward buttons (holding on the buttons will make video play fast (fast forward/fast backward) with no sound). In case the forward button or backward/rewind button keeps pressing by itself then try to clear data for Video app in Settings>Apps>'App name'>Storage>Clear data then open video app again to check if issue gone. You can Soft Reset your phone (no data loss) for further troubleshooting. Press and hold on Volume Down and power buttons together until phone restarts. Note: A screenshot may be taken while holding on the two buttons but keep holding on the two buttons until phone restarts.
