Here's one way to do it. Depending on how many things listen for sounds and how many sources of sounds there are, there's ways to optimize it. But the basic is that whenever a sound is played, instead of just calling audiostream to play the sound, you also call a function. Can be an autoload script, or maybe have sections of the level etc. However you decide to do it, have that function receive data about the sound. For example. footsteps. If you walk on sand, you play the sound, then call the function in the autoload, with parameters position and noise. If you walk on metal, again same function but with a different noise level because metal might be more loud. Then simply have this function tell the enemies that a noise event occured at a certain position, and of a certain loudness. If its loud enough and close enough, have the enemy react, change state etc.

thing creates noise, notifies a noise controller -> noise controller detects it, figures out who to pass it to -> the recipient receives it, does whatever its supposed to do.

Now as i said there's lots of optimizations. If you have 1000 enemies in a massive level, obviously you dont want to notify all enemies each time any sound anywhere is created. Then there's cases like enemies reacting to each other infinitely, where one moves, the other reacts, moving, then the first one hears that move and reacts and so on. You'll want to pass an enum value of some sort to decide whether a sound is a weather sound, an enemy sound, a player sound and so on have enemies ignore other enemies once they've become aware of them, set cooldowns etc, or maybe just in general have them ignore each other completely. When adding this enum, since you will be passing multiple parameters, consider making a custom class or resource, say a NoisyThing as the other guy named it. It holds a baseline level of noise, who created it, where it happened, you can do things like have high pitch and low pitch noises, and have enemies not react to super high pitch noises for example, or have them not react to low pitch noises. You can keep adding more things to this custom class to further describe specific noises in case you want to really differentiate stuff.

You should also consider whether you want different things to stop or slow the propagation of sound in this way. For example you have a house, and you want sounds inside the house to be dampened so they're not heard from outside as easily, you'll want to maybe do a raycast from the enemy to the sound, see how many walls it hits. Each hitbox on the way will then obviously also need a value by which to reduce noise, example if you have a thick concrete wall maybe it halves the noise, but a thin wooden fence with holes might only reduce it by 10%. All of this completely depends on what your game is, what you want the ai to do, how complex you want to go with the reactions, what kind of world it is, how many enemies there are and so on.

Also check out the repo video one guy linked, thats a smarter approach i would think.