Dishwasher soap - Baking Soda & Borax (all-natural household cleaner, in the laundry aisle) Mix together equal parts of each. Use just like a commercial powdered detergent, approximately 2 TBS per load. Each load will cost you pennies.
update: I tried with all my songs and yes I know it doesn't work with every song. I just think it works if you play the song on time or it works with some sort of upbeat song. But yea, it doesn't work perfectly like you would expect.