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

Well, once again you have an example of poor planning. But it looks quite well built from the pictures.

The system is too small for the size of the house (according to DIN still okay) and apparently only the minimum number of silencers were installed on the system. So everything is just enough to make it work without any margin.

Of course, the airflow rates will match the calculations, but in practice you get noise everywhere and the air in the house still feels stale. Everything you describe.

Of course, the air volumes will fit according to the calculation, but in practice you get noises everywhere and the air in the house still feels stale. Everything you describe.

Is there a rule of thumb – something like 40 or 50% above the maximum capacity over the demand – so that a system stays "in the green speed range" instead of "running flat out"? (I mean especially: in terms of an efficient performance – because what use is half refreshed air).

You can see in the data sheets at what load which sound pressure and also power consumption can be expected. I’d say 60-70% generally, the system should not be loaded higher under normal ventilation.

165 sqm ventilated area including overflow area * 2.5m room height * 0.5 air exchange rate/h = 206m3/h
The system of the original poster is already running at over 90% capacity at this point. Intensive ventilation is therefore no longer possible.

So I wanted to give a quick update. Someone from the company was here and put a foam plug about 10 cm thick into all the supply and exhaust air valves. He said these were the silencers. And it is really much better now. I still have to test the conversations with my husband tonight. But the loud noise of the system is now hardly transmitted at all. Do you think these plugs negatively affect ventilation? According to Alex85's calculation, the system is already at its limit anyway.

Depends on how everything is designed and what throughput rates are to be achieved. But since the system is already basically in the upper range without the dampers...

Yes, exactly those things are in there now. So in "normal operation" the system is set to 40% power. I assume that actually isn’t enough and that’s why the air seems/ is so used up. But when it’s at 100% it’s still very loud in all rooms. The installer said you never need 100%. Only if you’re having a party or something...