AN A319 CREW; INBOUND TO LAS; MISTUNED THEIR VHF COMS RADIO; MISSING CALLS FROM ATC.

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APCHING LAS; THE FO LEFT ARTCC FREQ TO ME WHILE HE WENT TO GET THE ATIS AND GATE INFO. WHILE HE WAS OFF THE #1 COM; I MONITORED THE FREQ AND DURING THAT TIME RECEIVED NO XMISSIONS DIRECTED TOWARD OUR FLT. THERE WERE OTHER XMISSIONS FOR OTHER ACFT DURING THAT TIME. WHEN HE RETURNED; I ASKED HIM TO GET A DSCNT CLRNC AND WHEN HE REQUESTED IT WE WERE TOLD THAT WE WERE ON THE WRONG FREQ AND INSTRUCTED TO CONTACT ZLA; WHICH WE DID. THE CTLR WAS NOT HAPPY THAT WE WERE NOT MONITORING THE FREQ AND ISSUED A DSCNT CLRNC. WHEN I LOOKED DOWN AT THE RADIO I REALIZED THAT THE FO HAD INADVERTENTLY TUNED THE #1 COM TO A DIFFERENT UNASSIGNED ZLA FREQ. ALL THE TIME HE WAS ACQUIRING THE ARPT AND GATE INFO; I WAS MONITORING THE WRONG FREQ WITHOUT KNOWING IT. I WAS HEARING OTHER ACFT RECEIVING AND RESPONDING TO CLRNCS AND BECAUSE OF THAT; DIDN'T REALIZE THAT THE SWITCH HAD BEEN MADE. THIS WAS THE SECOND TIME THAT THE FO HAD MADE A MISTAKE IN USING THE RADIOS. THE FIRST MISTAKE WAS NOT A TUNING ERROR; BUT AN IMPROPER USE OF THE SELECTOR BUTTONS ON THE AUDIO PANEL. WE DISCUSSED EACH INCIDENT AND I DETERMINED THAT HE KNEW HOW TO OPERATE THE RADIOS; BUT HAD SIMPLY MADE A MISTAKE. HE IS A HIGHLY PROFESSIONAL AND A VERY COMPETENT PLT WHO JUST MADE A MISTAKE.