I agree that the whole concept can be confusing. But to use it, you don’t need to understand the technical details. All communities can be viewed from any server (except if the server decides to block it).
In my opinion we should focus more on communities and less on where the community is hosted. How to do this, I don’t know.
What kind of sound do you want to soundproof? Is it the walking sounds or voices/music/TV?
Sounds like walking will travel through beams, walls etc and are best to be taken care of on the floors in the room above.