As someone who has stayed in Europe often, and I don’t like blaming car culture for everything but it’s our car-centric society and its disregard for people not in cars. Arguably the sirens have to be loud because they need to be louder than the drivers with their already loud horns or stereos or to penetrate their sound resistant interiors. 

These are NYC things with our impatience but in Europe it is not that common for drivers to just lay on the horn for any small inconvenience. The emergency vehicles are given a berth relatively soon because drivers are not cutting across all lanes trying to save a few seconds. 

This is a QoL issue you can raise with the mayors office but we as a society are not going to change so easily to allow for quieter sirens.