I know this isn't possible, but I still like the idea. It can happen in fantasy stories too.
What if a mage or a team of mages built the siren that's in my avatar. If you don't know what kind of a siren it is that is in my avatar, read my "Funky Avatar" post. Anyway, if a mage or a group of mages built it, I think it would be one heck of a magic device, loud enough and scary sounding enough to where a city could use it to scare off attacking armies without even a fight!! I can just see it now:
Invading army approaches a city that is equipped with one of these "Horns of the Gods" as we'll call it (airraid siren in avatar picture).
City garrison commander recieves report of approaching army and orders city mages to activate Horn.
When Horn is activated, produces terrifying howling sound that can be heard from as far as 4 miles away.
Invading army hears this noise, and turns tail and runs as fast they possibly can, scared out of their heads and fearing for their lives!!
I have got a few audio files of a few of these sirens actually sounding, but alas, I can't upload any of them here for obvious bandwidth reasons.