Have you ever been so scared you try to scream but all that comes out is a pitiful puff of air? Ever seen someone fall and get the breath knocked out of them so they lay there holding their chest, their mouth opened like their screaming but all you hear is a weazy woosh? It's pretty much that, use your context clues around the sentence and come up with your own conclusion, but a silence scream is just an attempted scream without the power to make it vocal.