Use USrangers sound pack and you will get a dive siren once you go faster than 300 true (I think that is the right speed). I use that sound pack and enjoy the siren, but now that I have put in more time flying the stuka, I find that very rarely do I get going fast enough to hear the siren before I have released my bomb and pulled out.

Personally I would rather see them update the 3D model for the stuka as well as add in the tank buster version,
before they add on a dive siren to the stuka we have now. (which didn't have the dive siren in real life anyway)