The pilot reported that while on the downwind leg of the traffic pattern, he selected the landing gear selector down but did not verify the landing gear was extended using the landing gear position indicator light. The pilot heard an unknown faint audible alarm as he approached the runway, which he later discovered to be the airplane’s landing gear warning horn. The pilot continued to the runway, and subsequently the airplane landed on its belly and slid off the side of the runway. The airplane sustained substantial damage to the lower fuselage. According to a mechanic who aided in the recovery of the airplane, the landing gear was found partially extended. He stated that during recovery operations, the landing gear extended freely by extending the emergency gear handle and moving the hand lever. The airplane was moved to a hangar and placed on jacks. The landing gear circuit breaker was found tripped. The landing gear linkage was re-connected from the emergency extension, and the circuit breaker was reset. The landing gear was cycled multiple times with no anomalies found. The landing gear warning horn and position indicator lights functioned normally. The pilot stated that if he had checked the landing gear position indicator before landing, he would have manually extended the landing gear. Additionally, he had never heard the landing gear warning horn before the accident.