Every word penetrates through the windows. Is it the same with you?

We have a solidly built house. With several floor-to-ceiling windows in every room. If someone stands outside at night in absolute silence and talks, we can hear it faintly but cannot understand it. During the day, we only hear delivery trucks slowly passing by in front of the house.

5cm window gap is simply a defect, apart from that it must be checked whether the windows were installed by mal assembly.

With us Ytong wall. Monolithic. You can hardly hear anything. No matter which window. I think that's normal. The windows fit quite precisely in the masonry with us. Karsten

The explanation sounds plausible and would fit well with our site manager...

This has less to do with the site manager, more with the architect. Normally, the architect should write in the plans that dimensions are to be checked on site. However, for that there logically must already be the wall opening if you want to order the windows. Therefore, some smart alecks behave as if you could simply add target gap dimensions and tolerances. Larger gaps could easily be closed with foam extravaganzas, knocking away stones when reality turned out to be narrower than the plan would take more effort. Unfortunately, sound does not take the foam remotely seriously.

I want to get a feeling for it.

You get that automatically when the sloppy work is fixed: the difference feels roughly like between window open and window closed.

That already depends on the site manager, because he also completed the drafting work, among other things. He had that as a secondary competence due to his original training. In that respect, he was responsible for both....

What can I refer to? RAL? DIN 4108? Or what counts?

For the purpose of the notice of defects, it is relevant that you specify symptoms. The specialist contractor must know which standard to apply.