A speaker needs a AC or a pulsed DC to make a sound, a buzzer only needs a DC.
A buzzer has it's own resonator. It works in most cases to feed the buzzer with a pulsed DC, but the sound will not be as loud as with pure DC.
There seem to be boards where the BEEPER-pin is not able to handle a PWM. Obviously intended for a buzzer.
To make these board able to handle a speaker
* replace the PWM based tone()-function again with a on-delay-off-delay loop.
Hopefully the last time I touch the beeper code.
Scott Lahteine