A B737-300 WHILE ON A STAR AT FL180 THE DSCNT CHKLIST WAS NOT COMPLETED AND ALTIMETERS WERE NOT RESET. INCURRED ALTDEV.

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FLT WAS ON THE GOSHEN THREE ARR INTO MDW. CAPT WAS PF. APCHING FL180 THE STANDARD APCH DSCNT CHKLIST CALLOUTS WERE MADE. DSCNT WAS CONTINUED TO MEET THE 10000 FT XING RESTR AT AWSUM. APPROX 1 MIN AFTER LEAVING AT 10000 FT; ZAU CALLED AND SAID 'CHK ALTIMETER SETTING 29.43.' MY ALTIMETER WAS RESET FROM 29.92 TO 29.43 AND A CLB WAS MADE BACK TO 10000 FT. THE CTR CTLR DID NOT INDICATE THAT ANY CONFLICT HAD OCCURRED DUE TO THE ALTDEV. HOW DID THIS HAPPEN? FIRST I WOULD LIKE TO HIGHLIGHT AN ACFT HARDWARE ISSUE THAT I BELIEVE WAS A FACTOR IN THIS INCIDENT. A PERCENTAGE OF OUR OLDER 300 SERIES ACFT HAVE AUDIO CTL PANELS THAT DO NOT ALLOW VOLUME CTL OF THE #2 VHF RADIO TO AN ACCEPTABLE LEVEL. BY ACCEPTABLE I MEAN TO A LEVEL THAT WILL ALLOW US TO MAINTAIN A RADIO WATCH ON COMPANY FREQ SIMULTANEOUSLY WITH ATC. I PERSONALLY TRY TO MAINTAIN THE #2 VHF VOLUME AT A RELATIVELY LOW LEVEL COMPARED TO VHF #1. THIS INSURES THAT ATC IS IN PRIORITY BUT YET ALLOWS ME TO MONITOR COMPANY DURING ALL BUT THE BUSIEST OF ATC TIMES. THESE 'PROB' ACFT DO NOT ALLOW ATTENUATION OF THE #2 VHF VOLUME TO A LEVEL THAT GIVES ME A LEVEL OF COMFORT KNOWING THAT IF I AM MONITORING VHF #2; THAT I WON'T MISS CRITICAL ATC COM. THE ACFT WE WERE FLYING; NUMBER; WAS ONE THAT I COULD NOT REDUCE THE VOLUME OF THE #2 VHF TO A LEVEL THAT I AM NORMALLY COMFORTABLE WITH WHILE LISTENING TO ATC. THIS IS HOW I FEEL THIS INCIDENT HAPPENED. I WAS MONITORING BOTH ATC AND COMPANY ENRTE. WE WERE GIVEN A XING RESTR AT AWSUM OF 10000 FT. APCHING FL180 I STARTED THE STANDARD CALLOUTS AND STARTED TO REACH FOR THE ALTIMETERS. AT THAT MOMENT ANOTHER COMPANY ACFT STARTED TO MAKE AN IN RANGE CALL TO MDW AND DURING THIS XMISSION ZAU CALLED US TO INFORM US ABOUT TFC. BECAUSE THE #2 VHF VOLUME WAS OVERRIDING CTR SOMEWHAT; I REACHED BACK (BEFORE GETTING THE ALTIMETERS RESET) AND PUNCHED OFF THE #2 VHF SO AS NOT TO MISS THE TFC INFO. THE FO ACKNOWLEDGED THE TFC AND WE BOTH SEARCHED FOR THE TFC MOMENTARILY. TOTAL TIME FOR THIS TO ALL HAPPEN WAS ONLY A FEW SECONDS. WE THEN CONTINUED WITH THE APCH DSCNT CHKLIST FLOW; BUT THE ERROR HAD ALREADY BEEN MADE IN THAT I DID NOT RESET THE ALTIMETERS. THE FO ACCOMPLISHED THE APCH DSCNT CHKLIST BUT FAILED TO NOTICE MY ERROR. ANOTHER POSSIBLE CONTRIBUTING FACTOR WAS THAT IMMEDIATELY FOLLOWING THE COMPLETION OF OUR APCH DSCNT ACTIONS; I WAS 'DINGED' BY THE 'A' FLT ATTENDANT FOR SOME SEAT BOTTOM CUSHIONS AT MDW. BUT I FEEL THE INITIAL EVENT THAT STARTED THIS SEQUENCE WAS MY REACHING BACK TO DESELECT THE #2 VHF IN ORDER TO HEAR ATC AND THEN THE MOMENTARY HEADS OUT LOOKING FOR TFC AND THEN FAILING TO START THE ENTIRE PROCESS OVER AGAIN.