1. Place your sounds in Steam/steamapps/common/left 4 dead 2/left 4 dead 2/sound. Weapon sounds are in the "weapons" folder, music goes in the "music" folder etc. They must be .wav files encoded at 44100khz. Otherwise, the Source engine cannot play them.
2. Start Left 4 Dead 2, but don't load a map. Open the console and enter "snd\_rebuildaudiocache". You game will freeze for a minute or so.
3. When it unfreezes, type "snd\_updateaudiocache". Now restart L4D and you're good to go!
If you make any minor changes or additions to your sound folder, enter "snd_updateaudiocache" again to quickly update your cache. Don't worry; that one takes literally seconds.