1. Navigate to Steamapps/common/Left 4 Dead 2.

2. Right-click > new > folder. Name it "custom".


3. Open "custom", create a new folder there and name it "sound" (left image).
This acts like a mirror image of your left4dead2/sound folder (right image), so put any custom sounds you want in here. Matching duration to the original sounds is not important, but I think the bitrate (mostly 44100hz) and number of channels (stereo or mono) is.


4. Run Left 4 Dead 2, open the console and enter "snd_buildsoundcachefordirectory ../custom".


5. You'll see a loading screen as the sounds are cached.
Once it's finished, it should look like this.


6. Exit or restart the game by entering exit or _restart

You should only have to build your custom sound cache once. I think it gets automatically updated when you change things.

7. Navigate to steamapps/common/Left 4 Dead 2/left4dead2.
Locate gameinfo.txt and open it with a text editor (Notepad++ recommended).


8. Add the following at the top of SearchPaths:
"Game custom"


Finally, save the TXT file. Now you can launch L4D2 and play with custom sounds!
Remember that when you verify local files (and sometimes when the game updates), gameinfo.txt will reset and disable your custom folder, so keep a backup handy.