Prefabricated house, poor sound insulation

and also that it is loud

What is loud? What is "too" loud? Under what boundary conditions? Under what structural conditions?

louder than normal

See above. What exactly is "louder than normal?"

The world is not just black and white. The trend is indeed to reduce all circumstances to these two states, but that almost always backfires. Especially with not quite trivial topics like building physics and building acoustics. Nobody would think of doing a BD measurement with an air pump and a wet finger and then say the house is "not tight."

The task was to evaluate whether a window replacement is beneficial. And I maintain that he can measure himself whether a window replacement is beneficial by finding out whether the sound comes through the pane or the wall.

How can I find out if it is the windows or the wall structure?

How can I find out whether it is the windows or the wall construction?

how about putting your ear to the wall and your ear to the window? works great!

whether a window replacement makes sense by finding out if the sound comes through the pane or the wall.

Of course, you can identify and classify the installation defect or improperly executed component connections by putting your ear to it. And then, of course, fix it by means of window replacement to a higher sound insulation class (SSK).

Windows are always the weak point, as they only have 30db Rdw., walls usually have 40 and above. Where do you get your knowledge about the complaints from? Did I overlook that?