We have the wrong version. The Siren was used on the 87-B. By the time the D rolled around the Siren had lost the psychological impact that it had in the first few months of the war. It was then removed. If we ever got the B, then it would be modeled, but I see no need for that version as we already have the D and the G for the tankbusters.
As has been mentioned, if you look in the Custom Sounds forum, you will find many sound packs that include the siren. I personally have it included in the soundpack I am using (Don't ask which one, no idea. Though I think it might be Frudas)