Step 1: Finding parts
3.5mm audio jack
led (any colour)
270 ohms resistor (red,violet,brown,gold)
s9014 transistor or equivalent
( http://www.datasheetcatalog.org/datasheet/auk/STS9014.pdf ) datasheet for the transistor.
Step 2: Construction
Solder the + leg of the led to the collector on the transistor and the - leg to the emitter.
Also add a length of wire to the - leg of the led for the ground wire.
Then solder the ground wire to the 3.5mm jack and the positive to the left or right channel.