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.
Step 3: Finished
I've found this wont work with some mp3 players with low volume but it works on laptops and tv's
they also make great badges and if you add another 3.5mm jack you can listen and watch it flash although i havent tried it and it might distort you music a bit.
If you want it to flash more lower the resistor value, I have mine so it flashes to singing but if you put in a higher ohm resistor you could make it only flash to bass.
Another idea is to make two and have one for music left ouputt,and one for music right output i imagen that could be quite fun to watch
Doesnt work ? :
try turning up the volume on the music sorce
check connections on all soldered parts
make sure you havent put the transistor or the led the wrong way around