The file extension only has access to shared storage…
EDIT: this statement is correct only for non media files
as we are talking about media files you should be able to get a list of media files from all accessible directories from your device, you will need READ_EXTERNAL_STORAGE permission for this, see also the overview
what about trying to access only the directory /Music?
Does the jukebox example work for you? Downloadable from here App Inventor Extensions: File | Pura Vida Apps
Taifun