Sound insulation of a downpipe passing through the living kitchen

Thank you! Then I probably should have paid more attention to the specific installation plans and their execution when buying from the developer. Or, if in doubt, just refrain from buying if such things do not count as defects.

It's really annoying but even though we were careful and checked everything 10 times, things still slipped through with us too ... You can now hear in the dining room when someone is showering upstairs ... Fortunately, that doesn't happen very often ....

Then I probably should have paid more attention to the specific installation plans and their execution when buying from the developer.

"If only" won't really help you.
If it's too disturbing, open everything that can be opened, cover the pipe with Armaflex, also cover the casing/the box from the inside, close everything up and make it look nice again.
It will be much quieter.
Whether you can get the developer to work or their general contractor, you'll have to see.

How would one properly seal the wall again? With a panel over it or, as it was probably done now, simply mortar on top?

How would you properly close up the wall again? With a panel over it or, as it was probably done now, simply mortar on top?

How is it now? Drywall - then drywall again. Or plastered - then plaster again.

The wall is currently plastered. Would the Armaflex then decouple the pipe from the plaster?

And would the angles of the bends (as far as I know, 2x 45 degrees) possibly need to be reduced?