on the toolbar in the top right click display folder content recursively, then it will also show sub folders files as well

I actually managed to do this but unfortunately can't really explain how in detail - I basically changed the views, then managed to see and select all the files, which then allowed them all to be changed from music to audiobooks.

I'm wanting to bulk change the "type" from "music" to "audiobooks" for many folders and sub folders.
The only way I can see to do this is to go into each audio book within it's own folder and change the file properties item. I can't select properties from any folder.
As I have many audiobooks I'm wanting to change this is extremely laborious - is there any way to do this in bulk?