The microphone bias from your sound card is only intended to deliver a few milliamperes of current to an electret microphone. It isn't a power source like you might think.
The bias voltage is connected to the microphone wire through a (fairly) large value resistor.
simulate this circuit – Schematic created using CircuitLab
Since the microphone draws a little current through that resistor, the voltage drops. You have multiple microphones, so they draw (together) more current so the voltage drops a lot more than usual.
If you had no microphones connected, but put an LED on the microphone input (paying attention to the polarity) then it would (probably) light up dimly.
It might not light up at all, though.
It takes a certain minimum voltage for an LED to "turn on." It varies by color. The lowest is the infrared (just a little over 1V) followed by red (1.2V.) Green and yellow take higher voltage, and blue takes the highest (around 3V.) If you have a white LED, then it is really a blue LED with some phosphor so it will take 3V as well.
That's all less than 5V, so you'd think it would work - but not all sound cards use 5V for the microphone bias.
So, you can't really add an LED in parallel to a microphone on a PC sound card. Even less so when you have multiple microphones connected.