This feels pretty obvious to me, though I have no idea if anyone else would share that sentiment. I was laying in bed last night, listening to the rain, when my town's own air siren started going off. The fire department controls it, and uses it to announce fires (I guess to call in the volunteer firemen or something).
And it occurred to me, listening to that siren blare in the night, that this sound is probably the exact same thing Alessa heard as she was being split in the fire in her home, and thus represents a significance to her via correlation with the event that made her supernatural. Ingame, the sound of the siren also accompanies a sinking into darkness as the Otherworld switches over to the dark side.

This can be used to further read into symbolism between the fire and Alessa's burning, and the behavior of the Otherwise and switching in the game. At first the town is relatively whole and normal, though nightmares still lurk around and the town definitely has some off elements, as did Alessa pre-fire. Then in a moment of drama the siren blares and the town switches over to a blackened, burned, ruined landscape, full of terrors and more dangerous monsters, making far less sense and seeming far less real.

The duality of Alessa pre and post burning can be compared to the duality of the two sides of the Otherworld.

Sorry if this concept has already been suggested and shot down by regulars, I looked in the quick links and the last page or so and didn't see anything.

yeah and it totally blares into the night sky and its echoes cascade the voluptuous breasts of the Appalachian mountain slopes

all you have to do is play the first 10 minutes of Origins to surmise this theory

I don't remember hearing the siren at all in Origins, actually. My memories of it are far more visual than audible.

You hear it when Travis is passing out infront of Alessa's house after he saved her. So yea, this isn't much of a "theory," more a "factual correlation".

The sirens Travis hears, however, are from the fire trucks.

maybe, maybe not

Well you hear the ambulance sirens going off and then in the midst of that you start hearing air raid sirens too I believe.

yeah, it starts to overlap and eventually drown out the firetrucks

This is true, but you have to remember who else was listening to those real fire trucks for the first time...

As a memorable part of her defining traumatic experience, it's only natural that Alessa would incorporate those real sounds into her nightmare world.

One of many insights that make Origins an interesting experience, and a worthy instalment in the series.

well i thought that's obvious.We had sirens in our town every month on each 15th but they don't do it anymore:(.it was so cool when you hear them...

I honestly never made the connection with the siren used by the fire department. I always presumed that the sirens just sounded to warn that the Otherworld is coming. Now that I think of it, it all makes perfectly sense.

They always test the air sirens once a month at 12 o'clock in the afternoon here in the Netherlands. It's always pretty cool when it happens

Sirens warn people of danger. That is really all the theory I've ever needed, be it ambulance, air raid or fire truck sirens, they are a warning of danger, pain and possible death.

Don't forget the air raid sirens used to warn that a tornado is coming. I agree, I take the siren as meaning "Something bad is coming, and you can't stop it. Run away! Run away run away run away run away run away..."

It's actually really scary, imagine being in the dark at a hospital and dark as hell except for the light coming out of your weak ass AA battery flashlight when loud as hell sirens are blaring out

I dont see much reason for Alessa wanting to warn anybody trapped in her nightmare world. The siren is simply something that echoes in her mind because she heard it at that terrible moment.

Lol, i remember that happened when i was sailing on het Veerse Meer

Maybe the Siren signals the coming of the otherworld, wherein a bloody, dark, fire-deteriorated place has been replaced

As stated before, I don't think the sirens (plot wise) are there to warn the character, but more a part of the over all shift from one hell to another.