Look at the wiring coming from the front of the case. There should be either a block or individual pins. If individual pins, then you should have seven total, three for the microphone and four for headphones. Out of those four, there should be RIGHT, RIGHT-RETURN, LEFT, LEFT-RETURN. Here's a 1 minute ASCII ART rendition of your pins
MP..G
M...P
R...RR
RS..B
L...LR
O...O
O...O
O = Optional
RS = reserved (actually not used)
B = blank, no pin
MP = mic power
G = ground
M = mic (second channel for stereo if available)
P = power (5v)
Here's what you're looking for:
R = right
RR = right return
L = left
LR = left return