Order-disorder

On my journey through Southeast Asia, I found myself in the jungle of Laos where I heard the gibbons (apes) for the first time. Their call reminded me of our air raid siren, which we faithfully test every first Monday of the month here in the Netherlands.

What does this call mean to the gibbons? Was this ever the inspiration for the air raid siren? If nature had an alarm, what would it sound like?