FLC OF AN MLG FORGOT TO RESET ALTIMETERS WHEN DSNDING THROUGH FL180 DUE TO DISTR BY THE LEAD CABIN ATTENDANT.

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DURING DSCNT; I FAILED TO RESET MY ALTIMETER SETTING TO THE LCL QNH. THIS RESULTED IN A 400-500 FT ERROR AT MY LEVELOFF ALT. THE LEAD FLT ATTENDANT CAME TO THE COCKPIT TO DISCUSS A CABIN INCIDENT THAT OCCURRED EARLIER. HE WAS WRITING AN INCIDENT RPT. THIS TOOK PLACE DURING THE EARLY TO MID PART OF DSCNT. PASSING FL180 THE FO SAID HE DECIDED NOT TO INTERRUPT OUR DISCUSSION BY CALLING OUT FL180 AND GIVING THE QNH SETTING; THINKING HE WOULD GIVE IT SHORTLY AFTERWARDS; SINCE OUR CONVERSATION WAS ENDING. HOWEVER; AFTER THE CONVERSATION ENDED; WE EXPERIENCED WHAT WE FIRST THOUGHT WAS A STUCK MIKE ON ZME'S FREQ. DURING THE COURSE OF TRYING TO COM WITH CTR AND LOOKING UP ADDITIONAL FREQS; I LEVELED OFF AT 12000 FT BUT WITH MY ALTIMETER SET TO 29.92 (ACTUALLY AT 12400 FT OR SO). THE MISTAKE WAS CAUGHT AND ALT CORRECTED BUT ONLY AFTER CRUISING AT 12400 FT FOR ABOUT 15 NM ON THE ARR INTO MEMPHIS. ALL OF THAT CRUISING TIME WAS SPENT TRYING TO REGAIN COM WITH ATC. ONCE WITH MEM APCH; APCH CTL TOLD US THAT ZME LOST ONE OF THEIR FREQS. NOTHING WAS MENTIONED ABOUT THE 400 FT ALTDEV.