The pilot reported that after four stop and go landings under night visual meteorological conditions, he planned to terminate the flight with a soft field landing. He further reported that as the airplane touched down, he heard scraping and the airplane subsequently slid to a stop on the runway. The pilot reported that he forgot to extend the landing gear for landing and did not hear the landing gear warning horn through his conventional (non-noise canceling) headset. The fuselage sustained substantial damage. The pilot reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. During a postaccident interview with the chief flight instructor of the operator, he reported that the gear warning horn in the accident airplane was normally audible in the cockpit, but does not sound directly through the headset. He further reported that they are in the process of installing a "supplemental voice" gear warning horn that will sound through the airplane's audio panel.