The NAS drive will need to be mounted to get the content in the Volumio “Server” side. Have you done this ? check out “Library” Menu tab if not.
However as you have a NAS drive it would make more sense to enable the DLNA Server or Apple Server on the NAS. What model of NAS drive do you have and is this functionality available ?
Then simply use Volumio as a “Client/Renderer” and Android/Bubble as your “Control Point”.
Some Advantages of this approach:-
-The NAS unit software is designed and packaged for file storage and indexing specifically.
-The files and the Index will be on the same unit (available to any device whenever NAS alone is powered up)
-Any future expansion to Multi Room will be straightforward, your server will already be established.