The hardware-components listed above were purchased individually and assembled by me in August of 2009 with the help and direct supervision of a friend who works in IT and builds several computers per month.
The Front Panel headphone jack on the Silverstone FP-34 is plugged into the AAFP jack on the back of the motherboard but the headphones did not play sound when plugged in. The motherboard's manual said to change the BIOS from HD AUDIO to AC97 to get this front-panel-headphone feature to work. (In BIOS Advanced Menu > Onboard Device Configuration> Front Panel Support Type > HD Audio-or-AC97.) Now the headphones play audio through the Silverstone FP-34 front panel jack---but the act of plugging in the headphones does not disable the external speakers.
So I got the headphones to work properly, but how do I auto-disable the external speakers when the headphones are plugged in?
That is a good question. It looks to me that you should be plugged into the AC97 plug on the mobo, since the audio that your supporting would be a simple AC97 and not the HD Audio, if I'm reading the Silverstone FP-34 ports right:
2 x USB A, 1 x USB Mini 5pin, 1 x 1394 6pin, 1 x 1394 4pin, 1 x audio (Azalia), 1 x MIC (Azalia)
This would explain you not having any sound at first, since you were plugged into the HD Audio pins and not the AC97 pins. Now I'm not 100% sure if you were plugged into the front panel that the external speakers would auto shut off. If you plug in your cable into the AC97 port on the mother and try it out, does it work correctly??
I'm pretty sure that if you plugged in the HD audio plug, you should set the BIOS to HD audio, rather than AC'97. I gather from what I've read that the automatic disable of rear speakers is part of the HD specification, not AC'97. Since both your motherboard and front panel connections are HD compliant, I would double check that you actually plugged in the HD connector. Otherwise, I'm surprised that the front connections did not work while the BIOS was set to HD.
A better question would be: "Why does ASUS put the "FRONT PANEL" audio connector at the rearmost bottommost point of the motherboard?!?!"