You should be using a sepqrqte power supply for the h bridge, you are not trying to run it from the arduino 5v pin are you ?
If so this isnt going to work. Check the two links related to servos in my signature below for a demonstration of the problem and the solution.
The link relates to servos, but its the aame concept, the arduino is great for control signals, but cannot power servos, motors, speakers etc the solution is to have a separate power circuit which cqn be as simple as a few batteries.
Duane B