I understand where you are at with this, I'm going to start off with a bit of math to show how it's not workable.
Commercial airliners move quickly, even in a controlled glide one is going close to 3 miles (4-5 km) per minute.
To give people on the ground enough useful warning to perhaps find some sort of cover you would need to give at least 20 seconds.
This siren would have to be able to penetrate the walls of buildings and houses and cut through the background noise of a built up area at least a mile ahead.
This is a very powerful siren and you'll have the following challenges:
At close range you would deafen anyone nearby, injuring the people you are trying to warn
The mechanism to generate this siren would likely be large, heavy, and challenging to design into an aircraft frame
You would need a great deal of mechanical or electrical power to generate that amount of noise.
If you have no engine power you won't be able to sound the siren, and if your engines are working you won't need to as the engines make very good sirens
already There are very, very few cases where a siren such as this would save very few lives compared to justify the costs.
aviation.stackexchange.com/users/1791/GdD
No they don't.
If you think about it, it is very rare for an aircraft to crash into a built-up area.
People on the ground would have very little time to hear a siren from an aircraft approaching at a hundred miles per hour or more.
Anyone hearing it might be as likely to run into the path of the aircraft rather than run away from it.
aviation.stackexchange.com/users/1289/RedGrittyBrick
No they don't, crashes over land where the plane was stable enough to provide enough warning to the people on the ground are very rare.
The only incident that would have benefited from it which comes to mind is the gimli glider .
aviation.stackexchange.com/users/609/ratchet freak
No, and even if they did you would probably not hear it.
Engine noise should be enough to alert someone who is paying attention, but the truth is that the average pedestrian has no cognizance of unusual noises.
Also, the odds of hitting a pedestrian are extremely remote.
Most people move around in cars.
Unless you are landing on a park or a beach there will generally be noone around.
I did hear of one case a couple of years ago where a guy ran out of fuel and landed on a beach and killed a jogger.
The jogger didn't hear him because he was wearing headphones.
When you come down it all happens really fast.
A pedestrian, even with a lot of warning would have to be paying attention, be alert and react right away.
Most people are absorbed in what they are doing and a siren going off 1000 feet away and 300 feet up is not going to be on their radar.
aviation.stackexchange.com/users/1378/Tyler Durden
They did design one plane with a siren to warn people on the ground.
The aircraft was called the "Stuka"The pilot would diligently turn on the warning siren right before he nosed down and dove at 300 miles per hour to accurately release his bombs onto the people he was warning.
The warning siren terrorized the general public, so they stopped using it out of consideration for the bomber pilots, who didn't like getting shot down by the military employed to protect the public.
aviation.stackexchange.com/users/8092/Jonas
The recent crash of a Taiwan aircraft, got me thinking, do aircraft have built in siren, which can warn people on the ground of a possible crash to give them some headway.  If that is not practicable or feasible, then what solutions are feasible, or one should abandon the idea altogether and think of making planes safer in the first place?
Do aircraft have siren to warn public in case of a crash over land?
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aviation.stackexchange.com/questions/12947
aviation.stackexchange.com/users/1678/Firee
Planes do not have sirens to warn people on the ground when they are going to crash, however, the engine noise should alert them. Even if they did have sirens, people probably would not hear them and, if they did hear them, the plane would be coming down so fast, they would have little time to escape. Fortunately, planes do not often crash over built-up areas. No, aircraft do not have sirens. Aircraft crashes in built-up areas are very rare and when they do occur, it is very rare that the plane would be able to provide enough warning to anyone nearby. When an aircraft is going to crash, it is moving very fast and there would not be enough time for the people on the ground to hear a siren and take action. Moreover, if the engines are not working, they make a lot of noise and this should be sufficient to alert anyone listening, although the average pedestrian would not be listening out for unusual noises.
 If the engines are not working, they make a lot of noise and this should be sufficient to alert anyone listening, although the average pedestrian would not be listening out for unusual noises. No, aircraft do not have sirens. Aircraft crashes in built-up areas are very rare and when they do occur, it is very rare that the plane would be able to provide enough warning to anyone nearby. When an aircraft is going to crash, it is moving very fast and there would not be enough time for the people on the ground to hear a siren and take action.