Poor building acoustics can really undermine a good design. I think there are only about five rules of thumb to keep that from happening.
Building Acoustics 101
Thanks to Hollywood everyone thinks sound absorbing materials like you find on the walls of a recording studio are what makes the room soundproof. The fuzzy stuff on the walls of a recording studio is to kill reflected sound. It doesn't keep sound out - or in. What’s inside the walls, ceilings and floors does that, along with sealed doors and windows.
When you want to limit sound from getting in or out of a room, you need to eliminate sound transmission paths and make the enclosing structure dense. The dense-ness can come from brick, PAINTED concrete block, glass or extra layers of drywall. Glass while great at stopping sound needs extra attention around the perimeter. The typical aluminum framing has numerous gaps through which sound can travel. Concrete block is dense but porous; paint seals the pores.
Commonly overlooked sound transmission paths (air paths) are walls that are not sealed to the floor or roof above, walls that are not sealed to exterior walls, and floors that are not sealed to exterior walls. Contractors love to stop walls 6" above the acoustical ceiling. Without some other measures to stop the sound from just going over the walls, you might as well have cubicles.
The nature of building acoustics is that it is generally easy to make a room less noisy - add some fuzzy stuff. It is much harder to seal sound out of the room if it wasn't addressed during construction.
Even the ideal room enclosure for keeping sound transmission to an inaudible level can be undermined by building utilities -
Ducts often act like a sound conduit as well as an air conduit. You need sound baffles to eliminate sound traveling through the ducts. Three right angle bends in lined ducts that travel between rooms will make a big difference but it won’t stop loud noises without baffles.
An under-appreciated solution to sound privacy is a white noise system. White noise works great because it creates about 50 decibels of noise in the same range as speech. However, a lot of people object to the white noise solution initially, but within two to three weeks they will never think about it again. The real downside to white noise is its green-ness. Its not very green. It is more stuff that has to be manufactured; and, worse, powered all day long. The opposite of green.
A great way to get an education in acoustics is to hire an acoustical engineer for a sensitive project and ask him a lot of questions. That's what I did.