So the building I live in is really bad with sound. What I want is a way to soundproof my room so I can talk with my normal voice without upsetting the people who live above me. The floor above me is the 5th and they can even hear the guy playing guitar who lives in the 2nd floor. I'm usually on voice chat at night and especially if something upsets me and I raise my volume, the entire floor above me can apparently hear me. Is there anything I can do aside from making my room the way a radio studio is?
Hard to know without some details of how the place is built - if they hear the guitar player from 3 floors below, I suspect there's a ventilation-duct connection acting as a "speaking-tube". If it was just the ceiling, a "dropped" or "suspended" acoustical tile ceiling would probably help. If it is a duct connection, some type of acoustic baffle on that would probably help.
Plug the gaps around the pipes, to start. Possibly some sort of wall-covering as well as the already suggested ceiling-covering, if it's all brick and concrete.
Since you own the place... this is what I would propably try... in order
Seal off any penetrations and openings using small cans of spray foam, caulk or similar. This includes plumbing, HVAC and electrical. including around outlets and boxes for light fixtures.
If that's not enough... Add insulation to walls and ceiling. Blown in rockwool has very good sound absorbing properties. I believe spray foam does too.It's not cheap but only requires one or two holes per stud bay.
If that's still not enough... Contractors who work on commercial office buildings usually are familiar with sound proofing. There are a number of ways of doing it but in your case I think the easiest (relatively speaking) option is going to be to put up a new layer of drywall on all the walls and ceilings that are adjacent to other units with some sort of isolation between the two sheets. There are special channels and I think possibly some rubber membrane, I even remember this acoustical caulk too. It's been a while since I've looked into this so don't remember all the details,
Not sure what to do about the floor. If the insulation works you might be able to talk to the guy below you and see if you can add insulation to your floor through his ceiling.
2 & 3 aren't cheap but your walls will look like regular walls and not be covered in egg crate foam or similar.
You didn't say anything about the type of construction of the building. Most people on this forum are familiar with wood frame construction the way it is done in the US. You mentioned Greece in your profile, so I assume your building is a reinforced concrete structure with brick walls? I would not expect this type of problem in such a building, so either there is something very wrong, or you live in a different type of house. If you have concrete floors, you can absolutely add insulation. I lived in an apartment with brick walls once and added fiberglass insulation and drywall/gypsum board on the wall that I shared with my neighbor. It worked well, but it is not a small job. You could do the same thing on the ceiling.