There are multiple ways of doing this, but they depend on wherever you will have a large amount of interactions or a very dense population.
Unfortunately if you want your simulation to keep a good precision you will need to handle most events generated by all actors and find all/most actors requiring notification.
Cheap area collisions
Concerning the problem with awareness areas you can solve it using simple collision detection.
In a 2d world doing collision detection on circles is "cheap". For example the operation to detect if a point is in a circle only takes 2 subtractions, 2 multiplications and an addition. You don't need to do the square root as you can store and compare the square radius of your area with the square distance directly.
If you use the 2d circle in a 3d world it will basically work as a cylinder. It can be a convenient way of creating areas if height is not very important.
Events oriented radius
If the volume of events (firefight, movement...) is low you can try to detect each actor affected by each event. Your events will generate areas (where they can be heard).
This is the simplest method to implement, it's also the most flexible as you can pickup bullet/projectile impacts originating from actors outside of an area of awareness.
On the other hand once the volume of events increases you might have to either reduce the number of events triggering area scans or reduce the number of actors scanned per event.
-> You can also create small (event) zones where all events are registered together and processed in bulks (i.e. 2 bullet impacts and a foot step happening in one zone will use only one scan and be sent to all impacted entities). <-
Actor oriented areas
You can use the "awareness area" principle. IE if an actor collides with the awareness area (circle) of another actor you simply add the colliding actor to a list of potentially interacting actors. Depending on how your engine is built, you can then register for sound messages and other events originating from the actors in the list.
To check for visual contact, you can also do your visual scans only on the list of registered actors.
You don't have to check for awareness area changes every tick. You can do it from time to time, every 5 to 30 ticks for example.
If the lists start growing you can limit them to a maximum size. But then you will have to prioritize the actors to add/exchange in the lists.
Mixed approach
You can mix both approaches. You can register actors in awareness areas for events like foot steps, engine noses, etc... And other events (projectiles impacts, explosions, etc...) can trigger scans based on their importance. There is a much higher chance someone will hear a grenade explosion than a bullet impact, so the grenade explosion should trigger a more extensive scan than the bullet impact.
I would recommend you start implementing the event radius first. Once this one works and you are able to reduce/increase the precision of this method at will you can start implementing the actor awareness areas. This way you will be able to start moving some events to the second system.
Context pools
Not all actors need to be notified of all events. For example an Anti-Aircraft turret doesn't need to be notified about bullet impacts and foot steps around it, even the presence of ground units could be ignored for some units.
If you find many such special cases you can create different pool of awareness areas. An actor can make his awareness area active in multiple pools. For example you might want your ground units to react to ground units only while some missile and laser equipped units could react to air units as well. Aircrafts never need to be notified about ground impacts other than explosions...
Of course you don't need to create multiple "lists" for the pools. You can simply use a bit mask and set the right mask for each actor/area. This way you can filter with a simple or before checking the distance.
Aggregation
If you see the lists in each awareness area grow and memory is scarce you can aggregate awareness areas for groups of enemies (teams, squads, platoons, flocks, swarms...) as long as they remain close to each other. This way an entire squad can register for events or other actors/groups entering its awareness area.
Basically the group entity becomes a substitute/proxy for all scans while all members of the group are removed from the pools.
This principle can also be applied to all units inside a vehicle.
Proximity activations
If the server is really overpopulated with bots (and you really need to keep them all alive), then you can only activate awareness/AI on those within a special "activation area around each player". This way bots (or groups) inside one or more activation area keep themselves and their awareness area activated (in the collision pool). Otherwise their awareness area is removed from the pool(s) of active areas.
The frequency at which those "activation areas" scan for "awareness areas" can vary depending on the speed of the player. A player traveling at higher speed will trigger more activations than a player camping a zone (needed at least to activate roaming bots and deactivate bots who leave the area).
Also you can deactivate a player's activation area if he boards a vehicle and assign an activation area to the vehicle if it doesn't have one. This way 10 players traveling in the same vehicle don't need to have 10 activation areas.
Delegation
If you don't fear cheaters and other trouble makers you can delegate some of the event detection to the client application.
The server can periodically send a list of nearby actors (bots and players) which will be notified by client apps. The client application will have to do the scanning and event detection for all events generated by the player. The it can for example send the list of actors to notify about a shot, a bullet impact or foot steps.
This is an option, it can be valid depending on the kind of game you are making. This is a theoretical idea and I would not recommend doing this until you have full control over the delegated clients (bot servers or secured clients).
I hope this helps.
