Controlling volume of W8's Music app independently

Most of the metro-style applications which play sounds implement the ISimpleAudioVolume interface. It is allowed to set a session-wise volume, but only if the developer had made use of it in that manner.

From the documentation:

Typical audio applications do not modify the volume levels of sessions. Instead, they rely on users to set these volume levels through the Sndvol program. Sndvol modifies only the master volume levels of sessions. By default, the session manager sets the master volume level to 1.0 at the initial activation of a session. Subsequent volume changes by Sndvol or other clients are persistent across computer restarts.

Hence, unfortunately, unless the application itself has implemented session-wise controls, you are out of luck in this regard,