The flight instructor and the pilot receiving instruction were conducting an instructional flight and entered the airport traffic pattern with two airplanes ahead of them. Shortly afterward, the instructor directed the pilot receiving instruction to lower the landing gear and partially extend the wing flaps. However, the instructor noted that the number of radio transmissions between the tower controller and the airplanes in the traffic pattern caused some distractions. He stated that a GUMPS check was completed turning onto final approach. However, during the landing flare when the engine power was reduced, he heard a horn followed shortly by a grinding noise from the propeller contacting the runway pavement. The pilot receiving instruction reported that upon entering the airport traffic pattern, he partially extended the wing flaps on downwind but did not extend the landing gear at that time. Due to the multiple radio transmissions and sequencing with other airplanes in the traffic pattern, he did not recall hearing the landing gear call from the flight instructor nor did he have time to look at the checklist. He noted that it was late by the time he noticed the landing gear was not extended. After touching down, the airplane departed the runway pavement and came to rest upright in the adjacent grass area. The left outboard wing struck a runway distance remaining sign which resulted in substantial damage to the airframe. A postaccident examination conducted by a maintenance technician responding to the accident site determined that the right main landing gear appeared down and locked; however, the left main and nose landing gears were not. The cockpit landing gear selector was in the up position. When the airplane master switch was turned on, the landing gear warning horn sounded. Upon lifting the airplane, the left main and nose landing gears extended into the over center and locked positions. When the landing gear selector was moved to the down position, the hydraulic pump started running and the gear indicator lights showed green. The flight instructor stated that, in a “moment of complacency,” he failed to verify the landing gear position. The landing gear horn did not activate until the engine power was reduced to idle upon entering the landing flare due to the need to maintain engine power during the approach. Similarly, the landing gear autoextension system did not activate until the engine power was reduced to idle during the landing flare.