Timber frame construction - Creating sound insulation between rooms

You could also drill a hole of about 10cm into the OSB and then blow in cellulose or wood fiber. Afterwards, glue up the hole and cover it with drywall. But it is probably difficult to do yourself, unless someone rents out such a "blowing device".

When the OSB boards are screwed in, they should be removed and mineral wool should be installed. With nailed boards, it becomes more difficult. Then only blowing in remains. Gypsum fiber boards are also better than regular gypsum boards. However, one can also use screed boards, but they are also relatively expensive and thick.

I unfortunately cannot say anything about the insulation between the walls. We have stone wool in there, but what was not considered in our case and what you might still be able to do is to make sure that the light switches and sockets on the adjacent walls are not installed at the same height/level. We can hear very clearly when the light switch next door is operated or a plug is inserted. There are apparently better sockets, but it is certainly even better to avoid this from the start.