I am trying to do this, light a LED from the headphone wire(mobile) but it seems it gives ac in there, I don't have much idea, please help.
If you must light your led without an external supply you could consider a small audio stepup transformer.Perhaps something out of a dead transistor radio.Otherwise you could build a diode pump that say quadrupled the voltage.Use shottkey diodes for your diode pump because they waste less voltage .Use a red led for your experiment because they need less volts than the other colors.
Yes, audio signals are AC. Headphone voltages will be too low to light an LED.
I don't really know for sure, but I would be checking out the mic wire. It could have bias current designed to power electret mics. Easy enough to check - just measure the mic to GND voltage, and then to be sure put in 1k - 10k resistor there, and measure voltage again. From these two measurements you should be able to determine open circuit voltage of that source, and internal resistance.
I assume you by headphone wire you mean the 3.5mm jack of your phone.
First consider lighting a LED. To light a LED you need a DC voltage and a few mA of current. Your audio jack isn't built to source current or provide DC voltage. It just transmits AC signal to your headphones.There is no way in which you will be able to light a LED from your jack.
You cannot use it for a purpose which it is not intended to serve.