I believe it is valid to challenge (not whine) the game developers for failing to impliment some fairly basic funtionality that could reasonably be expected by most users of a WW2 flight sim. Weapons have sounds, plane engines have sound, trucks & vehicles have sound, it seems rather odd that the air-raid sirens don't have sound
Maybe what's needed is a script, something like:
1. On missionload find all siren objects & record their x,y location & army (say in an array)
2. Ontickcount (say every 5 seconds)
3. Loop through the siren array: If siren is not destroyed & an enemy airborne plane is within Xm of the siren location, then play siren sound, else stop playing siren sound.