A PA34-200 INSTRUCTOR AND STUDENT LANDED WITH THE LNDG GEAR RETRACTED. GEAR UNSAFE WARNING HORN MALFUNCTIONED.

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THE VFR INSTRUCTOR CTLED INST APCH CONDUCTED IN VMC WAS CONCLUDED 2 MI S OF ZZZ RWY 2 AT 600 FT AGL OVER THE RIVER. CFI DID NOT RETURN TO FAF TRANSITION CHKLIST AND VERIFY THAT COMMERCIAL PLT HAD COMPLETED THE TRANSITION CHKLIST WHICH INCLUDED PUTTING THE GEAR DOWN. THE COMMERCIAL PLT HAD NOT PUT THE GEAR DOWN. LNDG ON RWY 2 AT ZZZ WAS WITH A L QUARTERING XWIND WITH A VELOCITY OF 5-10 KTS. AT THE INTENDED POINT OF LNDG; A SLIGHT DRIFT TO THE L DEVELOPED; FOLLOWED BY A TOUCHDOWN L OF CTRLINE. THE GEAR WARNING HORN WAS NOT HEARD PRIOR TO TOUCHDOWN. THE ACFT WAS SECURED AND BOTH PLTS DEPARTED THE ACFT. APPROPRIATE NOTIFICATION OF THE GEAR UP LNDG INCIDENT WAS MADE. DURING THE FLT; NO MANEUVERS WERE COMPLETED THAT WOULD HAVE INDICATED THAT THE GEAR WARNING HORN WAS NOT ACTIVATING AT THE APPROPRIATE LOW PWR SETTING. A DIFFERENT INSTRUCTOR PLT WHO FLEW A TRAINING FLT IN THE ACFT EARLIER IN THE DAY RPTED TO THIS CFI AFTER THE INCIDENT THAT; DURING THE EARLIER TRAINING FLT; UNLESS THE THROTTLE WAS AT AN IDLE PWR SETTING OR NEAR IDLE WITH THE GEAR HANDLE UP; THE GEAR WARNING HORN DIDN'T ACTIVATE. REGARDLESS; I; AS CFI; AFTER MAKING THE RADIO FREQ CHANGE FROM APCH TO ZZZ CTAF AND MAKING THE APPROPRIATE RADIO CALLS SHOULD HAVE RETURNED TO THE BEGINNING OF THE FINAL APCH CHKLIST AND AGAIN COVERED EACH STEP TO ASSURE THE COMMERCIAL PLT RECEIVING INSTRUCTION HAD COMPLIED WITH MY EARLIER INSTRUCTION TO COMPLETE THE TRANSITION PORTION OF THE CHKLIST; WHICH INCLUDED PLACING THE GEAR HANDLE DOWN AND ASSURING THAT THE GEAR WAS DOWN. THIS; REGARDLESS OF THE FACT THAT THE GEAR WARNING HORN DID NOT ACTIVATE AT THE APPROPRIATE LOW PWR SETTING. NEVER ASSUME THAT THE GEAR WARNING HORN WILL ACTIVATE AT THE CORRECT LOW PWR SETTING.