if you move the ground connection of the speaker in Michaels schematic to Vcc, you will get more volume.

assuming you have Vcc = 5V

Also replace the 1k resistor with a resistor calculated as follows:
(10/maximum Iol per pin)
The 10 should be a 5 but I use 10 to allow a safety factor.

maximum Iol per pin can be found under absolute maximum ratings in the datasheet for your derivative.

Erik