I've had a look at the plugin code for windows and for the media source it looks like you can specify a url, a 'file://' or a 'cdvFile://'.
The problem with the latter two is that they have to refer to either temp or persistent storage; it's not possible to specify that you want the audio file retrieving from a folder relative to the app root.
I've manually copied over my audio files to:
...and confirmed that they do play if I specify a path of: ''cdvfile://localhost/persistent/my_audio.mp3''
I've overriden the src/windows/MediaProxy.js code and hardcoded in various different paths for "thisM.node.src = srcUri.absoluteCanonicalUri;"
thisM.node.src = "ms-appx:///audio/my_audio_file.mp3"
thisM.node.src = "ms-appx:///my_app_name/www/audio/my_audio_file.mp3"
thisM.node.src = "audio/my_audio_file.mp3"
The first two don't report any errors when trying to play but nothing is heard.
The last one converts the source to:
... and reports an invalid src error when attempting to play.
So, it looks like something in the C# code also need changing.
As a work around I'll have to (if possible) check if the media files exist in the persistent folder and if not copy them over from the app audio folder.
Alternatively I could install the media files into the persistent folder when the app is installed but I haven't found a way to do this yet.