First of all, thanks to OP for reminding people here who are sensitive to loud sound.
Now to the movie. This is not a traditional narrative film. It is war, and the war is noisy. Less noise, and you'd be pulled back from the "circle of action" a bit more, and that's not what the filmmaker intended.
I had the privilege of experiencing "Dunkirk" at an IMAX theater with 70mm film projection and found the sound levels to be appropriate. I suppose it depends on the cinema, and nothing to do with Christopher Nolan or the sound designer. Great movie, great visuals and sound, and enjoyed every bit of it. Can't wait to see it at my home theater room which is recently equipped with a 4k projector , 135" screen and a good sound system. BTW, I didn't wear ear plugs during the movie as I am not suffering from tinnitus anymore. I never wore ear plugs even when I had tinnitus as my tinnitus did not react to sound.
I saw Dunkirk before I messed up my hearing and I can assure anyone else if they were wondering it's plenty loud. And this was before I had hearing loss.
I watched Interstellar at home on Blu Ray 2 years ago, and I remember it being mega loud. So I can imagine how much more louder Dunkirk will be at the cinema. Christopher Nolan's movies are always loud.
I watched it on my 65" oled in UHD - the sound ne levels signals in the soundtrack is set far higher than what the regular settings are in other similar UHD movies such as Hacksaw Ridge which is another recent war movie .
The quiet scenes are normal or on the quiet side but stukas attacks sirens sound are very loud pretty sure those weren't like that in real, or the pilots sitting in here would have been deaf
Overall an excellent movie if you like action and war flicks - and no signs of special effects. beautiful 4K details and color palette on the disk .