Questions about Sociology Writing Sample
I would not suggest starting from scratch. I am assuming you do not yet have publications, (but if you do, you should definitely submit the paper that has already been published in an academic journal). If not, don't worry. The schools you are applying to are aware that you have not done any graduate level work before and they are probably not expecting you to deliver a published or publishable article, but rather your understanding and ability to convey your ideas clearly in writing. If the papers you wrote previously are relatively well written, you can refine/improve one of them and have an outside reader (not necessarily a professor, but it would surely be useful) to read it and give you feedback. 

While you are refining your paper, you might want to look at published articles in Sociology and try to use them as a model to demonstrate your familiarity with the general disciplinary conventions.