There's a good dal ove overlap here in the 'Audio' section of MSS vs. gnome-settings-sound; which takes effect? For what apps? One should probably die. (ignoring s-c-soundcard, which also should get folded in somewhere.)
Found a related upstream bug.
"Multimedia Systems Selector" menu item should die. It should work out of the box now, bar bugs which should be fixed. I'll add this patch to gnome-media if it's not accepted upstream yet: http://bugzilla.gnome.org/show_bug.cgi?id=408292
We don't show the multimedia systes selector anymore, so I guess this bug can be closed for now. Further capplet reorganisation will happen upstream