You cannot add subviews to a toolbar. You must add a UIBarButtonItem whose view is the view you want to show.
UIBarButtonItem* b = [[UIBarButtonItem alloc] initWithCustomView: myVolumeView];
myToolbar.items = @[b];
This has nothing to do with MPVolumeView. It's true of any arbitrary view you'd like to add to a toolbar or nav bar.
Here's actual code that works on my device for a view controller in a navigation interface:
MPVolumeView* vv = [[MPVolumeView alloc] initWithFrame: CGRectMake(0, 0, 150, 40)];
UIBarButtonItem* b = [[UIBarButtonItem alloc] initWithCustomView: vv];
self.toolbarItems = @[b];
self.navigationController.toolbarHidden = NO;
Note too that you must test on a device; there's no interface for the volume view in the Simulator. :(