Insulation does most of the heavy lifting. Standard fiberglass batts in the walls and ceiling cut down outside noise significantly. If you need more, mass-loaded vinyl between the drywall and studs is the next step up. Solid-core doors seal better than hollow ones.
Seal every gap. Sound leaks through the same places air does: around doors, windows, and where walls meet the floor and ceiling. Weatherstripping, acoustic caulk, and proper door sweeps handle the small stuff. For most people, insulation plus sealed gaps is more than enough to take calls, record audio, or just work in peace.
