Does soundproofing improve or harm sound?

I am building a house and because of electrical mods, the GC knows I am an audiophile. He asked if I wanted soundproofing. I have never had soundproofing and don't know 1) if its effective; 2) if it will harm sound quality or improve it or not effect it.

The room is rectangular in shape and is 12 X 16 feet. Any input is appreciated.