This is a very simple Musicator that runs off the USB port. In its basic form, it only uses 4 components plus a single white LED. The 'star' of the show is the LM431 'Programmable Zener Diode' - which costs under 20c, so our project can be made for about a dollar in parts!
This allows us to double it up and have separate Left and Right channels, as shown here and below.
Step 1: Some Sample Videos
As you can see, the circuit is very sensitive and is able to display even soft passages on low listening volumes.