I think it's possible to scream and run at the same time, but I think that the image of the young woman tearing through the forest and yelling at the top of her lungs is probably more Hollywood than reality inspired. In real life things happen like you freezing, running out of breath, confusion, or just flat out not being able to bring forth the sound that you want. Have you ever tried screaming in your dreams? Like when you're trying so hard to yell as loudly as you can but the only thing coming out is a little yelp that's barely above a whisper? I think that sometimes happens in real life, too. The one time I saw someone running in terror she wasn't screaming, per se, just kind of panting really hard and gasping, "Oh my god, oh my god," as she ran.