I finally got to the bottom of the java.io.FileNotFoundException: www/{audioUrl} issue.
The answer was linked in (NATIVE AUDIO) Asset not found IOS which said this.
Solution
Cordova Native Audio plugin expects an assets “www” folder. But in an Ionic Capacitor project it’s named “public”.
As the plugin has this value hard coded, the solution in the link below tells you to dive in the plugin source code inside each platform and replace “www” with “public”.
Alternatively add a symlink from the “www” folder to the “public” folder in the android main/assets location.
See this issue for more info: