[eradeejay]

This is because L4D/L4D2 don´t have good support for custom content.
We are waiting a fix since L4D sdk release (one year) and for some reason Valve don`t want to fix the damn game.
We're screwed with mods.
The only way to fix the custom sound is with snd_rebuildaudiocache or you can pass your prepared cache file to the client in .rar or .zip but the client have to uncompress manually (is more faster than rebuild)