It is quite simple to get the external media volume buttons to apply to both master volume and headphone volume simultaneously.
Go to > System > Preferences > Sound and select both Master and Headphne (using Ctrl + click) under Default Mixer Tracks.
That’s it.
(Trying BloGTK for posting this entry)
More on this here http://freemor.wordpress.com/2008/04/13/using-blogtk-with-wordpresscom
