Shoutbox

I just have a bunch of shotgun-style approaches. Here you are:

Is there anything playing on the rear channels at all? Maybe it's working, but only silence is being played. Try playing a surround track that you know well, or use a test pattern, and see if the rear channels are really missing.

A restart is normally required when switching from stereo to quadraphonic, if you haven't restarted your computer since configuring your audio device, do so.

Maybe your headphones or keyboard is broken, try plugging a pair of powered (to be sure) speakers into the keyboard, and directly to the computer. Also check if there is a volume control in the keyboard or headphones. You might also try your keyboard/heasphones plugged into the green socket to test it (since your green socket is working at the moment.

Also try the other socket on the computer if there is one, maybe it's just plugged into the wrong spot.