A DC9 FLC ON APCH TO MEM FAILS TO MAKE THEIR ASSIGNED XING RESTR 20 MI W OF GQE; AR.

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FLYING FROM OKC TO MEM; WE WERE GIVEN RADAR VECTORS WHILE ON THE GILMORE ARR INTO MEM. WHEN GIVEN DIRECT TO GILMORE; WE WERE CLRED TO CROSS 10 DME W OF GILMORE AT 10000 FT. WHEN WE WERE DSNDING; THE FO SAID WE HAVE SO MANY MI TO REACH 10000 FT. WHEN WE GOT TO GILMORE; THE FO SAID 'I GUESS IT DID NOT MATTER THAT WE DID NOT CROSS 20 DME FROM GILMORE AT 10000 FT.' THAT'S THE FIRST TIME I REALIZE WE BOTH HEARD A DIFFERENT CLRNC. ATC DID NOT SAY ANYTHING; BUT I FEEL THIS CONFUSION IS NOTEWORTHY. FIRST WE HAD A RADIO THAT FORCED THE FO TO TURN UP HIS VOLUME TO THE MAX AMOUNT TO HEAR ANYTHING. MY VOLUME EVEN WITH THE VOLUME ON THE LOWEST SETTING WAS TOO LOUD TO THE POINT OF HURTING MY L EAR. I USED THE OVERHEAD SPEAKER AND TOOK OUT MY EAR PIECE. (THIS IS THE FIRST TIME I HAD EVER DONE THIS.) I THOUGHT I HEARD THE CLRNC TO CROSS 10 NM W OF GILMORE AT 10000 FT. I WAS ON A COMFORTABLE DSCNT AND REACHED 10000 FT 15 DME FROM GILMORE BUT THE FO HEARD CROSS 20 DME W OF GILMORE AT 10000 FT. INSTEAD OF SAYING THIS; HE MADE SUGGESTIONS LIKE HOW MANY MI TO REACH 10000 FT. I DID NOT PICK UP ON THIS CLUE. THERE WAS NO INCIDENT; ATC NEVER SAID ANYTHING AND I STILL DON'T KNOW WHICH ONE OF US HEARD THE CORRECT CLRNC. I MAKE NO EXCUSES AND I REALIZE THAT THE RESPONSIBILITY RESTS SOLELY ON MY SHOULDERS. I LEARNED THE FOLLOWING LESSONS: 1) TO FIND AN ALTERNATE MEANS OF MONITORING THE RADIOS WHICH DOES NOT RELY UPON THE SPEAKER BEING THE PRIMARY LISTENING DEVICE; AND NOT TO TAKE OUT MY EAR PIECE. 2) I NOW BRIEF ALL FO'S THAT IF THEY SEE SOMETHING THAT DOES NOT LOOK RIGHT; TO SAY EXACTLY WHAT THEY FEEL THE DISCREPANCY IS INSTEAD OF MAKING SUGGESTIONS OR TRYING TO MAKE HINTS.