Controlling overall system volume from app?

I'm building an application in which I would like to let the user control the volume via the app (instead of pressing down the side volume buttons).

Is there a way to control the "overall" system volume from within the app? If not, I would be content controlling media player volume and phone (ringer + in-call) volume. I create my own local MediaPlayer object, so I think that s开发者_运维技巧houldn't be so hard to control.

Quick google search found this solution