The main problem is that acoustics are affected by everything within a room because sound from humans and most loudspeakers spreads in a somewhat special fashion. So your acoustics problem is usually:
1. Sound goes everywhere and it is not directed just to the places where ears are. This way the sound waves of me speaking also hits the ceiling in a million places and these reflected signals come at you with varying time delay and mix with the direct signal of my voice. If the surfaces of the room are hard, you will have those sounds bouncing around till they loose energy.
2. On top of that every room has particular frequencies that it resonates with to create resonances that make that specific frequency ring longer or the room seems to suck the energy out of that frequency. These are called the mosal frequencies of the room.
One remedy to 1 is to make big chunks room surfaces absorptive or to put more stuff into the room that scatters the souns waves earlier. So: carpets, special ceilings, curtains, acoustic panels or foam at the walls, etc. Tirns out human flesh is also quite good ar this, so packing the room with people also helps.
Another way to deal with 1 (if you can't touch the architecture) is to emit souns in a very directed manner only to where ears are. Nut that means microphones, a mixer, 5k€ per piece beamforming loudspeakers + personal with enough acoustic knowledge to place them in the right spot and steer the beam in the right way etc.
The room mode thing is almost luxury to treat by that point, but it typically involves changing the room geometry, and building buying specially tuned absorbers that are tuned and measured within the room. Depending on how low you want to go with the frequency of your acoustic treatment these absorbers become big fast and can take a third of the space in a room.
In my experience the main problem is that people building rooms treat acoustics as some afterthought, something expensive that you need to avoid to save money. In fact the planing stage is the cheapest place to improve acoustics. You could for example not decide to make a conference room with pure concrete walls/ceiling and a tiled floor, because any person should be able ro imagine the acoustics of that room.
I had to work in a newly gallery space that had worse acoustics than a church. When I asked the architect about the acoustics and why the delay times don't fit the purpose, the architect said for money reasons they ordered "a better looking car deck". The acoustic treatment of that space will cost way more than if they just hadn't decided on a barren ceiling and pure concrete walls.
The same points go for insulation between rooms. There is no magic here, just physics. If you want insulation you have to put mass between rooms and decouple/dampen their interactions. You either do that from the start by building things correctly or you do that afterwards by building another floating room in your existing room. In both cases you have to inspect painstakingly for acoustic bridges, sometimes a single nail touching the outer wall in the wrong way already bridges enough sound to make it noticable.
We know how to build good sounding and well insulated spaces. People just don't want to pay for it. We know how to reproduce speech in acoustically horrible places, people just don't want to pay for it. Acoustics is perceived as an abstract, high-brow, fancy classical-music-topic. That is, until people perceive how straining bad acoustics can be, then they want sudden magic solutions.