IR transmitter connected to phone audio jack
http://www.instructables.com/id/DIY-Infrared-transmitter-for-iPhone-iPod/?ALLSTEPS This explains it pretty well but I'll expand a bit.

Since the audio signal is AC voltage it has negative and positive portions of the wave. By tying the grounds together and using the left and right signals as the +/- for the LED, the LED sees the voltage difference between the channels. If one channel is positive max and the other is negative max, this gives you more voltage than a single channel to ground. You use two LEDs so when one is reverse biased the other comes on (forward biased). You use a specific audio file in order to make sure when one channel is high the other is low (one is just the reverse wave of the other). WAV file obviously to avoid the losses

You are just not getting enough voltage off of a single channel to ground. The LED would only be on half the time (not an issue by itself with high frequency), and the positive half is probably not high enough to meet the LED forward voltage requirement.