It would be fine to have a Volume Control at Android Smartphones in the Playback screen.
you have it, look at the lower left part of the screen
uuups …
o.k., I produced a painfull impression 
Sorry so much that I disturbed You with gimcrack.
I will find my glasses - promised.
By the way … It´s applaudable for what else You find time.
May be the juice with you 
I do lot of stuff in my days! But the time I enjoy the most is the one I spend with you guys and Volumio!
btw, with the app BubbleUPnP I even can use the volume buttons of my Android smartphone 
(at least for me this works)