Am I correct in thinking that the little sounders found on computer motherboards are not a true speaker, just a piezo sounder?
That depends on your specific computer.
Lakes:
This type
http://www.ebay.co.uk/itm/Mini-PC-motherboard-Speaker-New-UK-/390236132930?pt=UK_Computing_Speakers&hash=item5adbe2d642
Yeah, that's probably a piezo.