[Pottus]

1.)

Code:

#define MAX_DYNAMIC_CARS 500 // If this isn't already in your script, add it

There are 2000 vehicles max this should be MAX_VEHICLES.

2.)

Code:

 enum VehicleSirenInfo
{
	EmergencyCopSiren
}

No need for an enum here at all.

Code:

stock InAreaWithCopSiren(playerid) // This is to check if a player is in the area with the siren audio
{
	new Float:x, Float:y, Float:z;
	for(new i = 0; i < MAX_DYNAMIC_CARS; i ++)
	{
		GetVehiclePos(i, x, y, z);
		if(IsPlayerInRangeOfPoint(playerid, 100.0, x, y, z))
		{
			if(VehicleInfo[i][EmergencyCopSiren] != 0)
			{
				SetPVarFloat(playerid, "CopSirenX", x);
				SetPVarFloat(playerid, "CopSirenY", y);
				SetPVarFloat(playerid, "CopSirenZ", z);
				return 1;
			}
		}
	}
	return false;
}

Code:

https://sampwiki.blast.hk/wiki/IsVehicleStreamedIn

Wouldn't checking if the vehicle is actually streamed in first be a good idea before doing anything?
Use of pvars is never recommended and should not be used here.

Code:

public OnPlayerUpdate(playerid)

This system is NOT a use case for OnPlayerUpdate() WHY? you ask. Instead of having the players check let the vehicles with sirens on do the checking! It boils down to if you have 100 players and 3 vehicles with sirens on you are having all 100 players check all vehicles each update. It makes a heck of a lot more sense to have 3 vehicles check 100 players every second since 100 meters is such a relatively wide distance +/- 20 meters when it comes to activation should give unnoticeable differentiation when it comes to performance. Even in the best case scenario that all players are standing still so OPU is only called once a second.

Assume 2000 vehicles
Assume 100 players
Assume 3 vehicles activated

Current method - 2000 x 100 = 200,000 iterations
Revised method - 3 x 100 = 300 iterations

That is a massive difference so if you are going to use OPU make sure it is absolutely necessary.

Code:

format(link, 128, "https://falcon-host.org/uploads/siren.mp3"); // This is the siren which I made and merged, but you can change it.

I really don't like hardcoded links in FS you have to assume that any kind of link has expired before the script is even released. Yes this can be changed but if you are going to have hardcoded links define them at the top of the script so the user knows right away this is a configurable option.