CAPT INADVERTENTLY SWITCHED COM FREQ TO TWR PREMATURELY WHEN SILENCING THE GEAR WARNING HORN DURING APCH VECTORS RESULTING IN LOST COM WITH APCH.

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WE WERE ON APCH INTO LAS IN WX AT NIGHT. I WAS FLYING AS FO AND I WAS ALSO THE PF. LAS APCH CTL WAS VECTORING US S OF THE ARPT AT 7000 FT FOR AN ILS TO RWY 25L. INITIALLY WE WERE VECTORED ON A HDG OF 090 DEGS. THEN APCH SAID HE COULD NOT FIT US IN BEHIND THE EXPECTED ACFT AND TOLD US TO TURN RIGHT TO HDG 110 DEGS AND EXPECT TO BE SEQUENCED ONTO FINAL IN THE NEXT 'SLOT' AT THIS TIME WE WERE ABOUT A MI SE OF THE ARPTS AT AN ASSIGNED SPD OF 170 KTS. WE PROCEEDED TO A POINT ABOUT 20 MI SE OF THE ARPT EXPECTING A TURN TO FINAL AT ANY TIME. I STARTED TO GET CONCERNED ABOUT OUR DISTANCE FROM THE ARPT AND TO NON COM FROM APCH CTL ALTHOUGH WE COULD HEAR THEM TALKING TO OTHER ACFT. THEN IT OCCURRED TO ME THAT THE VOICE WE HEARD WAS NOT COMMUNICATING APPROPRIATE MESSAGES FOR APCH CTL. I REALIZED IT MUST BE TWR FREQ. I QUERIED THE CAPT AND WE BOTH REALIZED IMMEDIATELY THAT THE SELECTOR SWITCH FOR THE COM RADIO HAD INADVERTENTLY BEEN SWITCHED FROM APCH CTL FREQ TO TWR FREQ. THE CAPT IMMEDIATELY SWITCHED BACK TO APCH FREQ. AS HE DID SO APCH CTL WAS QUERYING US AS TO HEARING HIS FREQ. THE CAPT ACKNOWLEDGED WE HEARD AND BRIEFLY EXPLAINED WHY WE WERE OFF FREQ. APCH TURNED US IMMEDIATELY TO THE NW TO JOIN THE LOC. THE APCH WAS COMPLETED WITHOUT FURTHER INCIDENT. FACTORS CONTRIBUTING TO THE SIT WERE THE PROX OF THE RADIO 'ROCKER' SWITCH TO THE GEAR HORN SILENCING BUTTON. EVIDENTLY; WHILE THE CAPT WAS ANTICIPATING THE HORN AND PRESSING THE SILENCE BUTTON AS I REDUCED PWR; HE HIT THE RADIO SWITCH. AS FAR AS OTHER FACTORS ARE CONCERNED; ONCE AGAIN IT IS SHOWN THAT ONE HAS TO BE EXTREMELY VIGILANT WHEN ON RADAR VECTORS FOR APCH FOR LOST COM.