Seal all gaps, cracks, and penetrations with acoustic caulk
Add weatherstripping around doors and windows
Install mass-loaded vinyl on the wall
Add a second layer of drywall
Use damping compound between drywall layers
Mount drywall on resilient channels or sound isolation clips
Fill wall cavities with mineral wool or fiberglass insulation
Build a staggered-stud or double-stud wall
Add acoustic panels or sound-absorbing wall treatments
Use thick curtains or wall hangings on the wall
Move furniture like bookcases against the wall
Replace hollow-core doors with solid-core doors
Seal electrical outlets and switch boxes with putty pads
Treat shared vents and ducts with acoustic materials