i dont know if im right, but the dive sound (when you dive) "appear" only when you dive from 8000 meters...
is it realistik?
In fact I think it appears in the game when you are approching the maximum speed, but not sure.
You should begin hearing it a little before 600kph.
Talking about the dive sound mod?
It is dependant on the aircraft, but for most you usually have to be quiet high (5000m+) to hear it before you hit the ground. And usually starts about 200km before airspeed never exceed or when first bits of aircraft start falling off.
I didn't know till the other day there's already a siren on the stuka, You activate the airbrake and the noise comes on, this is without the mod
In MPH IAS, the dive sound up to 16000FT will activate when you are over 365MPH IAS, rapid gaining speed and at higher alts it will start around 270MPH IAS. This is because the dive sounds are based on TAS KMH which is different at all heights, faster at high alt. It works on all AC except rockets and jets
I think people are refering to two different things.
First, there is a dive siren on the early war Stukas (up to Ju-87D3). This was added simply for propoganda purposes and to instill fear into the enemies. In game it is activated by deploying the dive brakes.
The 2nd dive sound is the one made by prop engined aircraft in a dive when they are overspeed. The DiveSounds 2.2 mod simulates this in game. If an aircraft goes overspeed it may happen and this is more likely at higher altitudes (higher to fall from, more time to accelerate). Hence in game you usually don't hear them til you have fallen a few thousand metres.