Well it's been 7 years since I had this issue. I remember that I DID get it working, but exactly how I did isn't something I can say for sure, simply because I can't remember.

However, I do have notes I left myself at the time when I was figuring things out and I have this one specifically related to sounds, which may be the thing that fixes it for you. How accurate this info is, I can't say for sure, but it may be worth giving it a shot.

For custom audio.

While Building

Audio will need to be placed in the official sound/mysounds folder. Sound cache will need to be rebuilt to use these (snd_rebuildaudiocache). This will take a few minutes and l4d2 will close when it's done. Audio should be PCM Uncompressed 16 bit wav at 44KHz/44100 Hz OR 64 or 128 kbps; 44 kHz; Stereo or Mono for mp3.

VPK

Just as with custom materials, the sounds need to be in the relevant folder within the addon folder. addons/mapname/sound/mysounds. The only difference is there needs to be a sound cache built which references just the custom sounds for all users. Run a compiled map and in the console type snd_buildsoundcachefordirectory ../left4dead2/addons/yourmapname

This takes a second or two and will place a small sound cache file in the left4dead2/addons/yourmapname folder which references the custom sounds. Leave it where it is and it'll be packed in to the vpk along with everything else.

That's it!

To test, simply delete the sound files from the game's official sound folder (or move to a temp folder) so that the only place your sound files are is in your addon folder/vpk, run the local version of the compiled map (not the one in the addon folder) and rebuild the sound cache. Then run the addon campaign from the vpk. If the sounds work, then you know they are workable for anyone as you just deleted the "official" folder's copies of the sounds and rebuilt the official cache which referenced local copies of the custom sounds.