There seems to be no way to control the volume of individual Metro apps from a single interface, which certainly seems like a step back to me.
All you can do (in most cases) is select Settings from the Charms bar (Win+I) and click the speaker icon, or use the desktop volume control from the tray/notification area.
However, if an app does provide its own volume control you should be able to locate it by looking under its own Settings in the Charms bar.
Edit: Apparently, not only is the lack of a Metro audio mixer by design, Microsoft is also discouraging devs from including per-app volume controls as per this MSDN blog post:
There is no audio mixer in this environment as there is in the desktop. It still exists for desktop applications, but your app won’t show up here because we felt it was not a great experience to pop out into desktop to adjust relative app volumes. Instead, we are encouraging apps to not include volume controls. This way users are focused on the master volume control which helps simplify the entire volume experience as well – but that’s a different story.
(Note: Substitute "simplify" with "dumb down").