Which material is suitable for the shell construction (thermal and sound insulation)?

Nominally, these may still be "massive walls." But with a bulk density of under 0.6 and very thin remaining ribs, it is already cookie-like. At least it feels that way.

The masonry is not the main issue. Use filled Poroton bricks, they are great for sound insulation, 36.5 with perlite. Then you get around 48db.

Much more important are your windows. Normal triple-glazed windows have poor sound insulation. They only offer about 32 db. And that's only at high frequencies. Low frequencies pass almost unhindered!

So install soundproof glass at least level 4. It doesn't cost much more but has a big effect.

What do you mean when you say it doesn't cost much more? Can you roughly estimate a percentage compared to conventionally used glass?

200 to 300 euros per window.

Nominally, these might still be considered "solid walls." But with a bulk density of less than 0.6 and very thin remaining webs, it already feels cookie-like. At least it feels that way.

You seem to know your stuff. Right? So, we built with Unipor Coriso W07. Easily achieved KfW55, with an air-water heat pump and controlled residential ventilation as well as triple-glazed windows. However, we made the mistake of using the standard ones because the window manufacturer said that would be enough. You don't hear much, but today I would install better panes.

You seem to know your stuff. Or?

Do you want to insinuate something to me? Regarding soundproofing, I would like to refer once again to my previous post:

Parents-in-law built directly next to the ICE railway line (less than 30 meters away) with 1.8 sand-lime brick. It turned out great. Although the soundproof windows also make at least an equally large contribution, so you definitely shouldn’t skimp on those. .... By the way, the parents-in-law have 24 sand-lime brick plus ETICS. Together with the soundproof windows, it’s first class, inside you can only hear a rustling while next door in the old house (brick) you first had to stop talking until the train passed

For the stone you mentioned, I find 47 dB at 36 or 48 dB at 42 thickness. Sand-lime brick already offers 49 dB at 11.5 or 54 dB at 17.5. And 60 dB at 24. A two-shell combination of 11.5 and 17.5 together with mineral wool then results in around 65 dB. Anyone who does not hear the difference of almost 20 dB should perhaps urgently see an ENT specialist. Even the 13 dB difference to the 24 sand-lime brick is already more than noticeable.

By the way, this is the crumbly cookie (Unipor Coriso W07), I estimate in total it is just a little more than 10 cm of bricks