Front panel audio--educated guess

I've had this case for almost ten years now, and I'm finally going to put in audio hardware that can take advantage of the front panel audio connectors. The motherboard is going to be as Asus P6T SE, and it has the standard 5*2 audio connector. Since the case is so old, I'm pretty sure I need to run the connector (adjustable in BIOS) in Legacy AC '97 mode, so the pin assignments are:
1 MIC2
2 AGND
3 MICPWR
4 NC
5 Line out_R
6 NC
7 NC
8
9 Line out_L
10 NC
which roughly conform to the Intel spec.

The problem is the connectors on the case. I have two cables, each of which carry three wires: black, white, and red. Two of the assignments are easy: the headphone white wire will go to pin 9, and the red wire will go to pin 5. After that I'm guessing. My first thought would be
Mic White->1
Mic Red->3
Headphone Black->2

Anybody know this stuff well enough to make a better prediction?

Well, for the mic, just get out a circuit tester: the tip of the plug is the signal input, the ring is the bias voltage, and the sleeve is ground. The black wire is probably ground, so you just have to trace the other two wires to find out which is which.

Excellent suggestion. After putting together a paperclip, a disassembled flashlight, and a "honey, can you hold this right here", I have determined that the white wire corresponds to the tip. This sort of make sense if you follow the tortuous logic that 1) the colors should match for the mic and headphone, 2) the tip of the headphone is active in mono setups, 3) according to wikipedia white is the color for both mono and left channels.

The next question is which black plug gets the ground pin? Or should I hack it so that they can share that connection?

I'd just tie 'em together... both need to be connected, but it doesn't matter much where you connect 'em, as long as it's a ground. Check first... sometimes one or more of the 'NC' pins is actually a ground, or sometimes there's another ground somewhere nearby (motherboard screw, etc.).