Controlled residential ventilation - You can hear every word in the other room - Is this normal?

Hello everyone,
We had the final acceptance of our house two weeks ago. Now we have found that through the central ventilation system in the guest bathroom and utility room, you can clearly and distinctly hear every word spoken in the dining/kitchen/living area, as if you were sitting next to each other. Of course, this echoes through the open toilet door to the upper floor to the children. On the upper floor, the sound transmission through the controlled residential ventilation is significantly lower.
Can I still claim this as a defect after the final acceptance?
Thank you very much and best regards!!

That sound propagates through the air is initially normal. Ventilation ducts can hardly filter out a "sound track" either. The ventilation system is nevertheless not intended as a house phone.

Kann ich das nach der Endabnahme noch als Mangel geltend machen?

If you are not sure whether something is a defect, the general rule should be: better to complain one time too many and not to dawdle.

Can I still assert this as a defect after the final acceptance?

Yes, definitely. I would advise not to mention too many details (controlled residential ventilation ...). The defect is initially the poor (ha ha) sound insulation. You shouldn’t write too much about the causes, that can only go wrong. After acceptance, it is of course more difficult (reversal of the burden of proof). Therefore, it is advisable to consult an expert. There is no rush anymore; a few days don’t matter. Pure curiosity: Have you ever tried to close the controlled residential ventilation (for example, stuffing rags into the pipe) and thus narrow down the cause? In operation, the sound wave might also propagate against the direction of flow. And "around the corner" (the pipe is hardly ever laid straight ahead) and then via the central unit (I initially assume a typical central controlled residential ventilation). ... You can tell, I don’t believe in the controlled residential ventilation as the cause. What about the wall structure?

Silencer present?

At a proper company, someone comes by and takes a look. They will quickly realize that something went wrong. An expert is not necessary at first. If you immediately start swinging the lawyer's club, they won’t be motivated either. Maybe it's just a small thing missing and the issue can be solved in 1-2 hours. Normally, you don't hear anything from room to room, since in a central ventilation system the pipes run individually as risers to the utility room and are only connected there with T-pieces.

Thank you for the answers, Yes, I spoke to the executing company today. They will come on Thursday and take a look. But they also initially thought it definitely came through the doors. However, you can hear that it comes through the ventilation system (central). It sounds as if you were speaking into a pipe (which apparently is the case). The doors block the sound well. The wall construction is with Ytong on the ground floor and drywall on the upper floor, and all walls on the upper floor are reinforced with OSB. Oh yes, you can also hear the noise of the controlled residential ventilation, which is generated in the utility room, very strongly through the supply air vents. The controlled residential ventilation (from Nibe) is then set to normal. Do you also hear this noise through the ceiling outlets of the supply air?