The idea is that, instead of only using one channel from your audio cable, by using two transistors you can use both channels and wire them to separate LED strings. Here I only show two LEDs in each string, but I'd recommend at least 6 for each.
Also, I decided to use USB to power my circuit. No more hefty dc adapters clogging up your power strip! I included a helpful USB cable diagram to tell you which wires you'll be needing.
I did not include resistors in this diagram because that largely depends on the type and how many LEDs you want to use. If you need help determining what resistors to use, this is a helpful resource to bookmark: http://ledz.com/?p=zz.led.resistor.calculator
If you find a mistake in my diagram, please let me know right away so I can fix it.
Here's a higher resolution of the image: http://i.imgur.com/O0MPL.jpg