You already know where an enemy is if you hear them behind the wall, you don't need a cheat to tell you that there is noise coming from other side of the wall. The server also doesen't need to tell you they are behind it if they're sneaking. A game that allows zero home-in time sounds like a flaw in the game and something solvable on the serverside.
You can replace a playermodel with wider "sound coming from around here" if you want to make it even harder for a cheat to pinpoint a sound
I feel like you've only played 1 genre of video game or something.
> You already know where an enemy is if you hear them behind the wall,
You know they are somewhere behind the wall, you don't know which exact angle they are behind the wall, because headphones and our ears don't work with that degree of accuracy.
The cheater can just swing the corner with his cursor already pre-positioned exactly on his target. Between peeker's advantage (inherent to any online game with latency) and human reaction time, there's not a lot you can do to fight that.
> The server also doesen't need to tell you they are behind it if they're sneaking
This requires the server to calculate line of sight checks for every player, which is costly, requires loading the entire geometry into the server and would be horribly prone to latency. Then you're looking at potential performance problems on the client due to only knowing about a player the second its in view and having to stream the assets to the GPU, which if don't happen in time for the frame you'll experience as hitching.
> You already know where an enemy is if you hear them behind the wall
Yes but this requires using your brain rather than just seeing them straight up through a wall.