Just ordered the book, thanks Bob! This is why this forum is great.
As far as sound energy isolation, we really need to break it up into two areas; airborne, and structureborne. There are a couple of things to remember- everything resonates (materials are excited into vibration that then becomes airborne), and vibration is a two-way street. If sound can get in , it can get out. It can do so through air leaks and flanking, and/or through mechanical excitation. Note also that structureborne vibrations reflect back too. This might mean the shell back into the room, the floor back to the loudspeaker cabinet, etc.
When talking about the shell of the room, too much mass will help noise from transferring to other spaces, and sound infiltration from other spaces, but will also mean the sound energy will be more contained, be allowed to build up, and be allowed to linger more than we would like from a sound quality POV.
Shell construction is a delicate balance between noise control and sound quality. Both existing (or estimated) conditions must understood, as well as the goals for noise control and sound quality. Then with information of the sound energy paths, and any physical, budgetary, decor, etc. constraints understood, solutions can be considered.
Ideally, the shell is allowed to flex mechanically, yet is sealed resiliently air-tight. As always, acoustics is never a one-size-fits-all. Construction materials and methods and interior treatments will be needed- the right type, at the right location, at the right quantity.