Reduce sound transmission through chimney

A question about soundproofing. A chimney runs through the living areas of a newly constructed apartment building. Unfortunately, every noise and conversation can be heard through it, especially the sound transmits from the lower to the apartment above. The construction work is actually already completed, meaning the walls are plastered, and the floor with underfloor heating is installed. What structural measures can still be taken to reduce the sound transmission?

There is something wrong imho; there should not be an opening from two WHG into the same shaft. Have the responsible chimney sweep master check that. Otherwise, dampers should of course also be installed in chimneys.