So, do you have a test meter ?
If not, connect the jack plug to your sound card and run a free 'sound card oscilloscope' programme that will display the voltage.
Connect the red or green wire to the ground wires (make sure the insulation is removed from the wires).
Does the displayed signal voltage change ?
(It is easier to buy a very inexpensive Chinese-made testmeter).
Are you sure the microphone input of your sound card uses a stereo plug ?
It is more likely to be a mono jack plug.