Thank you Roger...for everyone who wants to add the siren to the exterior, put these two entries into the sound.cfg file, replacing the currently existing wind_sound entry:
[wind_sound]
filename=dive
flags=0
viewpoint=1
rparams=200.000000,1.000000,520.000000,2.500000
vparams=200.000000,0.000000,520.000000,100.000000
minimum_volume=4500
maximum_volume=9500
minimum_rate=0.60
maximum_rate=0.90
minimum_speed=200.0
maximum_speed=520.0
link=wind_sound.1
[wind_sound.1]
filename=dive
flags=0
viewpoint=2
rparams=200.000000,1.000000,520.000000,2.500000
vparams=200.000000,0.000000,520.000000,100.000000
This is very simply written, so feel free to make refinements if you deem them necessary, such as to the "r-rate" params and "v-volume" params.
In the lines that read 200.000000,1.000000,520.000000,2.500000 - this means that at 200 mph, the rate will be exactly what it is in the stock sound file, and 520 mph, the rate of the sound file will be multiplied by 2.5 times. These figures control the pitch of the sound you hear, so it will start out normal, and get ever so higher in pitch as you approach 520 mph.
In the lines that read 200.000000,0.000000,520.000000,100.000000 - this means that up to 200 mph, the volume of the sound will be 0, and by the time you reach 520 mph, the sound volume will be what I believe indicates 100% by that figure.