This sounds unlikely.

IR commands are modulated 38 kHz (ish) signals. If your phone could reproduce audio to, say ,10 kHz, which is WAY above what a phone needs to output, this is two octaves above that.

Second, LEDs are driven by a voltage that has to be more than probably two volts. IF (and this is an if) your average cell phone speaker is an eight ohm speaker, that would represent about a half watt of power, again, unlikely.

I don't see how the phone can use its audio amp to output such a signal.

See why we want to know why you're trying to do this? We might be able to work out a great bit of software that would still not do what you want because the beginning assumption -- that a phone's audio output will turn on an LED -- is flawed.