Prefabricated house, poor sound insulation

Hello, it is a new building

Hello, would it also help to install gypsum cladding on the interior walls?

Rather no.

The exterior wall consists of 80 mm polystyrene insulation boards, 12 mm OSB panels, 140 mm interior insulation, and 12 mm gypsum fiber boards.

What is this interior insulation made of and could it have sagged so that cavities now act as resonance bodies? - Have you already named the manufacturer here?

Hello, the cause is simply that only a single layer was applied on the inside. There is not enough mass present.

And why should gypsum claddings not have any effect? It's additional mass again after all.

Well, the mass of gypsum claddings is probably negligible

Well, the mass of gypsum veneers is probably negligible

One square meter of gypsum veneer weighs about 18 kg. It is significantly more than with a gypsum board.