You may try to connect the LED to an earphone jack and stick it in your phone. Then play music. Don't expect too much of it. The power you can draw is very low. I don't have a phone with an audio jack socket so I connected an LED to the earphone socket of a radio and it worked. (By the way, if you decide to have a try, you'd better test the rig on something you don't mind frying.) If you play a regular music file the light will flicker. You should download or create a file with such frequencies that your eyes won't be able to see the LED alternately light and dim. You may try PWM. Rectification of the signal may help to gain some extra power but at the cost increased complexity and cumbersomeness. I hope this may help you.