Adywan said :

no, i don't like the idea of adding a siren. the scene doesn't need it. he gave the announcement, which would have been heard city wide, so the need for a siren to signal the evacuation seems pointless.

I'm just worried about the shift workers who are having a kip while Lando delivers his announcement.

Bespin Worker 1: "Did I just hear the Baron warning us about the Imperials taking over the city?"

Bespin Worker 2: "Go back to sleep darling, it was just another one of those funky cheese nightmares"

Bespin Worker 1: "Did you put the cat out?"

Bespin Worker 2: "No! it's still burning in the oven"

Bespin Worker 1: "I don't know I've got a bad feeling about this"