Install thick, well-fitted blackout curtains
Add a heavy curtain liner or acoustic curtain
Seal gaps around the window frame with acoustic caulk
Apply weatherstripping to the sash and frame
Use a window insulation film
Install a removable acrylic or polycarbonate window insert
Add a secondary interior window pane
Replace single-pane glass with double- or triple-pane glass
Use laminated acoustic glass
Install soundproof window plugs
Close and lock the window tightly
Add draft stoppers at the bottom of the window
Fill any frame gaps with expanding foam where appropriate
Upgrade to tighter-fitting window hardware
Add exterior storm windows
Use dense window shutters
Combine multiple methods for better results