Can you try copying your files to:
C:\Program Files\Inductive Automation\Ignition\webserver\webapps\main\YourFolder
and using the url:
http://<Ignition IP or hostname>:8088/main/YourFolder/YourAudio.wav
This is how we store cached versions of map tiles locally without webdev, but i believe this might be wiped when upgrading? Not sure... However we recently did a patch update from 8.1.9? to 8.1.15 and these files weren't deleted.
@Paul.Scott had some words around this here:
I agree though that the web dev module is expensive when you have a very basic use for it