In Northern Ireland where I am ambulances only seem to turn their sirens on when approaching junctions where they will have to go through a red light, otherwise they just have their lights on

They'll also blip them when traffic isn't getting out of the way, but that also seems incredibly reasonable. I honestly can't remember the last time I saw a "needless" siren on an emergency vehicle. But, like you, I am in the UK and the rules are different here

In Munich, they seem to regard every crossing as dangerous. :(

It really is quite obnoxious.

In Germany police and emergency vehicles are only allowed to use "Sonderrechte" (special privileges) when using both the lights and the sirens. So if they want to pass a red light or go the wrong way in a one way lane they have to use the sirens. Note that this is not technically a legal requirement as the law grants them a general exemption but the administrative regulations make this mandatory.

That said, I have seen police cars go the wrong way using hazard lights instead of the blue lights (and no sirens) so I'm not sure what the actual consequences for not following these regulations are.

I also noticed just how loud sirens are in Berlin. They're far louder than what I've heard in other European cities. Does anyone know why that is?