Wearable Arduino mod for your headphones. Non-invasive procedure for headphones that do not have room inside drivers or you does not want to hack out your high-end cans.
Adafruit has just put a tutorial for the addition of model flashing LED indicators for headphones. This instructable takes a step further.
This project is the project Adafruit Amp-tie has turned into a cold reactive LED activate the device for your headset. It provides an audio-visual indicator, or a color organ that responds to the volume/amplitude of the sound heard in your headphones.