Simple LED Circuit on audio jack not working
It is most likely because the mic bias circuit does not supply enough current to turn on your LED>

Below is what a typical PC sound card input looks like. Your phone's mic input is very similar, but is biased at 2.7V (which is a typical voltage to run a cell phone's analog circuits) and the bias resistor and audio ride on the same wire instead of 2 shown for the PC. The circuit is from here: http://www.hobby-hour.com/electronics/computer_microphone.php



The 2.2k resistor provides a bias voltage required by the amplifier inside the condenser mic in your headset. On some phones, this resistor can be as high as 10k Ohms.

A red LED has a forward voltage of 1.8 Volts typically. Your supply is at 2.7V. So it looks like this:







simulate this circuit &ndash; Schematic created using CircuitLab

So let's do the math. In the best case, the internal bias resistor is 2.2k. Red LEDs typically have forward voltage drop of 1.8 Volts, so your LED current is (2.7 Volts - 1.8 Volts) / 2.2k Ohms = 0.182 mA. Pretty small. Repeat the calculation for 10k and you get 40 uA (0.040 mA). Miniscule!

Here's how you find out the value of the internal bias resistor without opening up your phone: set your multi-meter to the 20 mA range and then connect it between the mic and GND connection. Then use Ohms law to calculate R. So if you measure 1 mA, you would calculate: R = V / I = 2.7V / .001 = 2.7k Ohms.