I think a better solution would be to grant every player auditory perception based on his PE, but not hindered by walls. After that we would only need a sound that only plays when somebody is running.
Compared to t888 idea this is more realistic as you couldnt tell what weapon the enemy is wielding and the number of enemies would also have to be estimated.
Would it ruin running through doors and around corners one hexing?
But I am 2 or 3 int powerbuild ape RavenousRat, My 3 int can only afford few points in sneak, my 2 int can't at all :\
But if you want to implement something like this then I will suggest another thing to build to it: add some feature to a radio like item where you can type a command "/decoy" and then the radio will play a recorded gunshot sound for a few minutes and you can drop this radio behind a wall or corner, where other plays will see the image of some player shooting until they run around the wall and see it's just a radio on the ground with pink writing over it *bang bang bang*.
He eithers moves and you exactly what is he doing or he stops and you can be sure that hes in the same spot as he was before. I dont think walking will be used in large scale, fast-paced actions.
One more use for sneak skill, if "hearing" will be the same as current FoV, but much less radius and independent on directions, then not-so-high sneak skill may allow to you to stay unnoticed from "hearing" FoV and you still can ambush others, especially if your victim isn't 10PE+SS sniper. In other words, hearing FoV will have some penalty to max distance and work the same as current FoV, but ignore obstacles.