Adjusting Volume In Music Player Using SmartKey App

I am attempting to customise the SmartKey Android App with Volume Up & Volume Down settings so that I can alter volume whilst listening to music via the native 'Music' App on my Xperia Z3 phone. Please see attached image showing my customised SmartKey settings.

This does not seem to be possible.

I would be grateful to hear if it should be possible to use SmartKey for adjusting volume and how I can do so.

Many Thanks

John