Materials are important. Technique is more important. Before you go buying Etruscan marble because it has the highest STC rating, you should be realistic about effort needed to pull it off.
For isolation, one of the best conventional approaches is cinder-block filled with cement or sand. Of course, this is a whole different skill set from wood framed building, which can also be effective with double walls and proper structural isolation.
The aforementioned SAE pages have very good basic theory about construction.
If you're looking for plans and more detail, get the Alton Everest books. They're on Amazon and invaluable resources.
Good Luck!