First, dip a spoon in hot water and shake off the excess water. Next press the spoon against the a piece of dry ice. The warm spoon will cause the dry ice to sublimate, or turn directly into gas. The pressure of the gas will lift the spoon off the ice, but the pressure of you holding the spoon down will push the spoon back against the dry ice. This process repeats so fast that a singing sound is produced that varies with the pressure.
This was demonstrated on the David Letterman show a few years ago.
Make sure to follow all dry ice safe handling precautions.