Sudden death of at three year old ball python?

Given that the aspen was caked around her mouth, I would think that she had a lot of mucus coming out. That hissing or wheezing sound that you heard is a sign of a respiratory infection. With the conditions that you described (filthy cage, poor temperature control, etc.), it does not surprise me that she had a respiratory infection. If she had trouble breathing when you got her, the infection was already well advanced and would have required antibiotics to clear...and even that is a hit or miss thing with an advanced respiratory infection. Snakes can't expel mucus the way a mammal can, so it kind of just wells up and out through the mouth, nostrils and the heat pits. Without aggressive care, the mucus builds up and slowly suffocates the snake.