One would think: 5V/150ohm = 33mA, no additional resistor needed.
Speaker's are actually rated in impedance, not resistance, which means the effective resistance varies with frequency. Typically dropping with frequency. If you measure the resistance with a meter, you would be measuring at 0 Hz, you will see a lower number - add a resistor to that number to bring the resistance up to 150 ohms to keep your arduino output pin safe,