The flight instructor reported that the pilot receiving instruction performed a simulated single-engine approach. He wanted to be able to communicate more easily with the pilot, so he set the simulated failed engine to 15 inches of mercury manifold pressure since the landing gear warning horn will sound if the manifold pressure is set below 14 inches of mercury. He added that he did not verify the gear was down and locked with the three green extended lights illuminated. He reported that, just before landing, he heard the landing gear warning horn but attributed it to the stall warning horn. The pilot receiving instruction reported that he did not recall hearing any audible alarm. The airplane landed with the gear retracted. The airplane sustained substantial damage to both the wings. The flight instructor reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.