For instance, I only want to hear the "Message recieved" sound for my girlfriend, so I know to minimize my game or whatever I'm doing that might be full screen.  I know there are already custom sign-in sounds for specific contacts, and I use that already.

Also, first post, yay!

The suggestion was for message sounds, not sign in/out.  This has also been suggested previously (kind of):  http://shoutbox.menthix.net/showthread.php?tid=33991

See my post in that thread.  The suggestion is basically to have a "Message Sound" as well as Sign In and Sign Out sounds in the Notifications options in Plus.