The problem is that
Media Views are cascading filters, meaning that they inherit the filters applied to their parents:
Each Media View is essentially a saved search, just like a Smartlist, which then allows you to further sub-filter the results using Categories. These saved searches are cascading filters, meaning that the results of a parent View, also limit the results of its children. So, the reason that the views underneath Audio in the tree show only audio files, and not photos and videos, is that the top-level Audio view has a filter that says:
[Media Type]=[Audio]
And likewise for the Images and Video top-level views, each of which is filtered to show their particular Media Types. Since these rules "cascade" to the children of those top-level Views, each sub-view doesn't need its own copy of that part of the search.
So, if you put a View underneath Audio in the
Tree for Music Videos, it will
always be empty if you use the filter:
[Media Type]=[Video] [Media Sub Type]=[Music Video]It will always be empty because you're essentially saying:
1. List only Audio files. (This is the filter applied to the top-level Audio View.)
2. Of these files, pick all of the Video files (the [Media Type]=[Video] part of your Music Videos view).
Answer will always be zero files because you can't have a file that is both Audio and Video at the same time. It never gets as far as filtering it to only show Music Videos, because the potential list is already empty.
So, when you make a hierarchy of views, they always systematically "narrow down" the selection. For that reason, if you make your own View underneath Audio in the Tree, you don't need to include [Media Type]=[Audio] in it. That's redundant because it is already included in the top-level Audio View.
But...
In Standard View, you can make a particular Child View
not inherit the filters of their parents. In the Edit View dialog under Included Files, uncheck the box that says
Use parent scheme rules for file display. Now, that View will "start fresh" and show all possible files that aren't excluded by its own filters.
This option is not available in Theater Media Views or Network Media Views, so you have to resort to additional trickery to accomplish the same thing there. But for Standard View, you can just uncheck the box, and then search for whatever you want in the child View, and it'll work as expected.