Materials And Sounds
Save the contents of the sounds folders to:
Windows: C:\Program Files\Steam\steamapps\common\portal\portal\sound
MacOS: ~/Library/Application Support/Steam/steamapps/common/portal/portal/sound
Save the contents of the materials folder to:
Windows: C:\Program Files\Steam\steamapps\common\portal\portal\materials
MacOS: ~/Library/Application Support/Steam/steamapps/common/portal/portal/materials
(On Windows 64-bit, replace “Program Files” with “Program Files (x86)”. On older versions of Steam, replace “common” with your Steam username.)
You may be asked about overwriting files if you have downloaded other map packs, like Portal: TFV. It does not matter if you overwrite the conflicting files—they are the same.