I don't see how this is an operating system "problem" that needs to be resolved by a specialized design team.  Why not just put a volume control in every application?  A new mail sound isn't a "system notification," it's a sound that is played by Outlook.  Just put a volume control in Outlook.  It seems like all the extra sliders and the syncronization, the meters and etc. would be more confusing to a user than a simple individual volume control slider.

"One little thing can solve an incredibly big problem."