For this project you shouldn't even need to buy anything
A pair of speakers (or one, whatever you want)
1 bright led (I chose red because I think it worked the best, plus it's my only bright LEDs on hand)
wires for permanent installation, or alligator clips for demonstration.
Step 2: Open the speaker
Open up the speaker. Usually all the screws come out except one, so you just pull the case apart and it'll pop out.
Step 3: Locate the speaker
see that big metal thing? That's a speaker. If any wires are coming off of it (like another speaker) that's easier to reach, then just use that. I think mine was hook up to a tweeter? I'm not an audio guy, so I'm not sure.
Step 4: Attach
attach either a wire or aligator clips to each connector. connect the other ends of the wire or aligator clip to your led. Even though it's an led, polarity doesn't matter for this application since it's ac. No resistor is needed either.
Step 5: Crank it
turn you speakers up and play music until the led is reactive. It starts being reactive at about 25%, but then (for me) it was best at half as loud as the speakers can go.