Musical Underwear is the underwear that gets down when you get down!
(Video is NSFW if your work frowns on undergarment flashing)
Step 1: Supplies
Also you'll need a pair of underwear, i chose some awesome looking ones from American Apparel.
And lastly you'll need a phototransistor that reacts to visible light a mini audio jack with free wires (not shown) and a 3 AA battery holder