When I was umping (mostly youth baseball, U-14, U-16, etc.), I always found it pretty simple to line up your angle and get a view of the ball and the foot. It was a little trickier from the runner-on-first, behind-the-mound position, but still pretty easy. I also think the usual default call is out. Past a certain point, your eye has no idea anyway, and the sound never much helped me.