Lewis, please do tell how!
Clive sorry for the late reply but I was trying to figure out how it worked. I installed it a long time ago and couldn't remember exactly which files were used or how those worked. I was looking for the link that Rocketred posted to that alert siren download. After downloading and looking at that one I believe the one I have works basically the same but the file names are a little different. I didn't make it and I don't recall where I got it either. Mine has a gun named siren_gun, a facility file named siren, a wav file named b_airraid, a vehicle folder named siren, a text entry in the sounds.xml and a text entry in the ACC_Graveley_42.xml below are the file entries and file text strings. Unless I've missed something I think thats all there is to it. I double checked the alert in QC again and it works perfectly whenever an enemy aircraft appears over the field. I deliberately start off on the runway sometimes just to hear the siren go off at the approaching enemy aircraft. Of course Gravely is the only airfield I have it installed in right now but in theory it should work in any airfield you install it in.
Sound.xml
<!--CAS Air Raid Siren-->
<siren>
<sound file="b_airraid.wav" Looping="No" Atten="0" Stereo="Yes" MinDistance="50" MaxDistance="300"/>
</siren>
ACC_Graveley_42.xml
<Facility Type="Airbase" Flags="isAirbase" OuterDistance="10001">
<Unit Type="siren" Position="60.93 -217.22" Angle="0" />
siren.xml
<Facility Type="siren" Flags="" OuterDistance="10">
<Units>
<Unit Type="siren" Position="100.15 -1.35" Angle="0.00"/>
</Units>
<GroundPlanes/>
<Routes/>
</Facility>
siren_gun
<?xml version="1.0" encoding="utf-8"?>
<UnitData>
<General Allegience="0" LongName="" ShortName="" ModelName="" BlastDamageScale="1.0" ImpactDamageScale="1.0" FireDamageScale="1.0" Priority="2" Type="gun" EnteredService="01/01/1936" BlastDamageMod="0" ImpactDamageMod="0" FireDamageMod="0" AllowSpawn="n" Category="weapon" Country="neutral"/>
<Description String="Warning Siren"/>
<Loadouts/>
<GunStations/>
<DamageBoxes/>
<Systems/>
<Gun GunType="flak_gun" TargetCategory="aircraft" SoundName="siren" Range="50000" Rate="1" MuzzleVelocity="200" TimeAlive="500" Noise="0" DefaultWeapon="dr_blank_round"/>
</UnitData>
siren (vehicle)
<?xml version="1.0"?>
<UnitData>
<General Allegience="0" LongName="" ShortName="Siren" ModelName="siren.m3d" BlastDamageScale="1.0" ImpactDamageScale="1.0" FireDamageScale="1.0" Priority="4" Type="static" EnteredService="01/01/1930" BlastDamageMod="1" ImpactDamageMod="1" FireDamageMod="1" Category="wheeled" PickType="armor_aa_static" Country="neutral" AllowSpawn="n"/>
<Description String=""/>
<Moving MinSpeed="0" MaxSpeed="0" CruiseSpeed="0" GroupSize="0" Type="0"/>
<Loadouts/>
<GunStations>
<GunStation UpLimit="90" DownLimit="0" LeftLimit="360" RightLimit="360" RateLimit="360" SystemID="engine_one" Tracer="0" Trainable="1" Trigger="0" Type="siren_gun" Name="Siren" ConvergeDistance="0" Pitch="0" MaxAmmo="10000"/>
</GunStations>
<DamageBoxes>
<Box ID="damagebox_nose" Parent="damagebox_nose">
<BoxMap SystemID="engine_one" Probability="100" Points="100"/>
</Box>
</DamageBoxes>
<Systems>
<System ID="engine_one" Name="">
<Threshold Level="80" Effect="Break" Parameter="" Location=""/>
<Threshold Level="100" Effect="Library" Parameter="fx_grndexpl_m" Location=""/>
<Threshold Level="100" Effect="Library" Parameter="fx_damage_vehicle_l" Location=""/>
<Threshold Level="100" Effect="Break" Parameter="" Location=""/>
</System>
</Systems>
<Effects/>
<Ground RoadMove="0" WaterMove="0" ForestMove="0" MtnMove="0" CrewSize="4"/>
</UnitData>