I don't think players focus on the sounds of a players footsteps to notice someone. Generally seeing the player is enough
Stealth rating sounds pretty neat, instead of just getting the full stealth benefits from having one stealth item on, you'd only get partial benefits which grow as you add more and more stealth items.
Hiding should be permanent for all levels of players, it'd be better for level to dictate the proximity in which you're completely invisible and the degree of lessened transparency you undergo as a monster/player nears you. Of course that transparency change should be unique to that player/mob