The problem is that the inter-room partition was built without filling the interior space, without rock wool inside. One room has already been repaired. Actually how to fix the situation with noise isolation without destroying the wall?
Maybe throw a soundproofing membrane and another layer of plasterboard on top, or just a few layers to add fireproof gypsum? There is enough space, you can take another 5-10 cm from the wall for finishing. Maybe build a new frame of high quality?