38kHz sounds tricky but it is actually easier than you think, provided the line output is high enough.

LEDs need some minimum voltage to light up. Imagine an audio line playing a sine waveform and an LED connected between the live end and the ground (remember a protective resistor, but the audio output's internal impedance should suffice in this simple scenario). At the points where the waveform level is below the threshold, the LED will be dark. Where the level exceeds the threshold, it will be lit up, like this:



Now connect a second LED anti-parallel to the first one. It will act exactly the same as the first one, only on the negative part of the waveform:



However the receiver does not know that the light comes from two LEDs and not one. It knows nothing about the original sine waveform either. All it can see is light pulses.

Note that the light pulses come at a frequency that is double the frequency of the original waveform:



You can double the frequency again (to 4x the original waveform) by using the above arrangement in e.g. the left channel, the same on the right channel (i.e. 4 LEDs altogether) and playing the waveform through the two channels phase shifted by 90°.