Lived in altbau buildings for 10+ years. It totally depends. In our previous flat the most silent wall was the (modern) drywall to the adjacent apartment. In the one before there were steal beams which transmitted noise if something like a speaker was positioned too close.

The worst buildings are probably the postwar period buildings which were built both cheaply and without the regulatory framework in use today.

I lived in a very quiet building from the 50s in east Germany. But my apartment was at the top, so no noises from above, and only quiet people to one site.