The noise can be minimized by someone light on his feet (including his 131lbs) and in good shape
The girls had no reason to worry about being heard.
One of the order fillers/workers Roy Lewis said this in "No More Silence."
"Sometimes if you were on one of the floors by yourself somebody would sneak up and you'd never know they were there.....They could walk down the stairway and a lot of times they'd be on you before you'd know it..."
What's the evidence that the steps made loud noises or were loud when you went up and down? I can understand multiple people making noise but Lewis's account says one person could go down quietly. And that's in a quiet time or atmosphere; there was lots of noise outside during the aftermath of the shooting.