Do aircraft have siren to warn public in case of a crash over land?
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.