There are a lot of broken audio add-ons in the Workshop. For example: Santa Christ Theme Song Tank Music

To fix that add-on:

1. Use GCFScape[nemesis.thewavelength.net] to extract the root folder of the vpk file.
2. Rename the root folder to "SantaChristTankTheme".
3. Create a folder in there named "sound".
4. Move the music folder into the sound folder. Dragging it should work.
5. Go into SantaChristTankTheme/sound/music/tank
6. Use Audacity[audacity.sourceforge.net] to export the .wmv files to PCM .wav files.
7. Delete the wmv files.
8. Drag the SantaChristTankTheme folder onto (a shortcut to) "C:\Program Files (x86)\Steam\steamapps\common\left 4 dead 2\bin\vpk.exe" to create a working vpk file.
9. Put the new vpk file into "C:\Program Files (x86)\Steam\steamapps\common\left 4 dead 2\left4dead2\addons"
10. Start the game and make sure your add-on is enabled before any conflicting ones to make sure its changes aren't overridden. Test whether the volume of the custom sounds is high enough and that all textures are working. (Make it easy: Enable the development console in the options, toggle it with the ` key, and enter "sv_cheats 1; god 1; map <mapname>". "maps *" will show you a list of maps.)

To create your own mod, use one as a template and replace/delete the files. For non-workshop mods, you should add addonimage.jpg (128x128 pixels) next to addoninfo.txt

I've tested these steps on 64-bit Windows 7. File locations may vary. On other systems, Wine[www.winehq.org] might help, or open a terminal to run vpk <vpkfile> (That will create a folder next to the vpk file.) and ffmpeg[www.ffmpeg.org] or somesuch.

According to the fun but buggy Wand Massacre addon, sound files are assigned per level, e.g. chapter 10 mission 1 defines sounds in WandMassacre\maps\c10m1_caves_level_sounds.txt

"Melee.Wand.Miss"
{
"channel" "CHAN_WEAPON"
"soundlevel" "SNDLVL_NORM"
"rndwave"
{
"wave" "weapons/wand/miss_1.mp3"
"wave" "weapons/wand/miss_2.mp3"
"wave" "weapons/wand/miss_3.mp3"
"wave" "weapons/wand/hit_wall_1.mp3"
"wave" "weapons/wand/hit_wall_2.mp3"
"wave" "weapons/wand/attack.mp3"
}
}

That label is used in the weapon script: WandMassacre\scripts\melee\wand.txt

// Sound data
// define the hit and miss sounds
SoundData
{
"melee_miss" "Melee.Wand.Miss"
"melee_hit" "Melee.Wand.Hit"
"melee_hit_world" "Melee.Wand.HitWorld"
}

The sound files are at:

WandMassacre\sound\weapons\wand\miss_1.mp3
WandMassacre\sound\weapons\wand\miss_2.mp3
WandMassacre\sound\weapons\wand\miss_3.mp3

More info can be found at https://developer.valvesoftware.com/wiki/L4D2_Custom_Sound_and_Music_Tutorial