On April 8, 2001, about 1730 hours Pacific daylight time, a Piper PA-24-250, N6240P, sustained substantial damage when it made a wheels up landing on runway 26 at Chino, California. The commercial pilot/owner was operating the airplane under the provisions of 14 CFR Part 91. The pilot, the sole occupant, was not injured. The personal flight departed Corona, California, about 1720. Visual meteorological conditions prevailed and no flight plan had been filed. This occurrence was upgraded to an accident on April 19, 2001, after an inspection revealed damage to structural airframe components. The Federal Aviation Administration accident coordinator interviewed the pilot. The pilot said he initiated a go-around on his first attempted landing because the runway wasn't clear. He forgot that he raised his landing gear. He was wearing a new noise attenuating headset that changed the sounds he normally heard when flying his airplane. He did not recognize the gear warning horn during the accident landing. He recalled that the gear warning horn was still sounding as he removed his headset.