Instructions
Open your "GMod" installation directory, found on your main hard drive in the file path "Program Files/Steam/SteamApps/garrysmod."
Open the "sounds" folder in your "GMod" directory. Copy the existing sound files out of the "sounds" folder to a convenient location on your computer.
Place the new sound files into the "sounds" folder in your "GMod" directory.
Close the file and load "GMod."