I'm trying to do this
http://www.instructables.com/id/Music-LED-Light-Box/step1/Materials-Tools/ project with a TIP31AG NPN Transistor and a 9v Battery as my power source. The circuit works fine, but it's not sensitive enough. It reacts properly, but only when I turn the volume to about 80% of what my iPod can put out (pretty uncomfortable). The guide didn't mention a specific type of transistor -such as TIP31C instead of TIP31AG, so I think the gain on my transistor may be too low. (This behaves the same way regardless of how many LEDs I have attached, be it a single LED or nine)
My experience with this stuff is limited to AP Physics in high school, so I don't understand much beyond resistors and capacitors.
I have some random resistors on hand, along with a few other transistors, 2N3904, 2N304, 2N2222, etc...