A piezo speaker has a high impedance, whereas a regular speaker's is very low. I'd be afraid of damaging the mobo by trying to drive what is essentially a short to ground with a circuit designed to oscillate with a piezo. If you have a high-value resistor around, or a small capacitor, you could alligator-clip that to the mobo pin and connect the other end to an amplified speaker lead, like from a desktop stereo setup. The speaker ground would go to the other pin. You might hear something, and all you want is post beeps.
Maybe you could dig the piezo out of an old cell phone or other piece of equipment that beeps, like a dead camera or that Christmas card your brother-in-law sent you that plays "Grandma Got Run Over by a Reindeer" when you open it.