FLT CREW DISTRACTED BY COCKPIT NOISE AND CONFUSING ROUTE INFORMATION; DESCENDED TO 5000' WHEN ASSIGNED 8000'.

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WE WERE OPERATING LTT X IFR MIA TO HRT. NEAR TLH WE WERE GIVEN DIRECT TO CEW FL350; IN THE HI LEVEL STRUCTURE. APCHING CEW WE WERE ISSUED A DSCNT ORDER TO AN ALT IN THE LOW LEVEL STRUCTURE (FL180). MY F/O PUT ASIDE HIS US (HI) 5-6 CHART AND OPENED UP HIS US (LO) 31-32 CHART. THIS IS CORRECT PROC SINCE MANY LOW ALT AIDS AND FIXES ARE NOT IN THE US (HI) CHART. NEARING 10000' WE WERE GIVEN A NORTHERN VECTOR; A DSCNT AND INTERCEPT A JET ARWY. SINCE JET ARWY ROUTES ARE ONLY SHOWN IN SHADOW ON US (LO) CHARTS; MY F/O HAD TO RECLAIM HIS US (HI) 5-6 CHART. HE UNDERSTOOD THE ARWY TO BE J5 AND TRANSPOSED THIS TO 5000'. THE F/O READ BACK 5000' AND THEN SET OUR ALT ALERTER TO 5000'. IT WAS NOT UNTIL WE MADE OUR PROPER RPT ON REACHING 5000' THAT WE WERE MADE AWARE THAT THERE WAS SOME QUESTION ABOUT THE ALT WHICH WE THOUGHT WE HAD BEEN GIVEN. THE CTLR NEVER QUESTIONED OUR DSCNT FROM 10000 TO 5000'; NOR WAS THERE ANY WARNING OF CONFLICTING TFC AS WE MADE OUR DSCNT THROUGH 8000 TO 5000'. I KNOW THAT THE F/O IS VERY CONSCIENTIOUS AND METICULOUS IN HIS FLT PLANNING AND COCKPIT PROCS. HE MAY HAVE MISUNDERSTOOD IN THE SCRAMBLE TO TRANSIT FROM THE US (HI) 5-6 CHART TO THE US (LO) 31-32 CHART; THEN BACK TO US (HI) 5-6 CHART IN ORDER TO CONFIRM THE CORRECT ROUTE ISSUED. I HAVE OVER 40 YRS AND 15000+ HRS OF SAFE FLYING WITH NO VIOLATIONS. IN MY OPINION THIS INCIDENT COULD HAVE BEEN PREVENTED IF THE CTLR HAD BEEN CONSISTENT IN HIS STRUCTURE CLRNCS AND NOT SWITCHED FROM HI ALT TO LOW ALT; THEN BACK AGAIN TO HI ALT STRUCTURE. WE WERE ONLY SEVERAL MI FROM CEW AND A DIRECT TO CLRNC WOULD HAVE BEEN MORE APPROPRIATE. I RECOMMEND THAT ARTCC PROCS USE ONLY LOW ALT AIDS AND FIXES ONCE ACFT UNDER THEIR CTL ARE IN THE LOW (BELOW 18000') ALT STRUCTURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: BOTH REPORTERS RECEIVED NOTICE IN WRITING THAT THIS INCIDENT WAS UNDER INVESTIGATION; WITH THE FAA ALLEGING THAT THEY DEVIATED FROM AN ASSIGNED ALT OF 8000' BY DESCENDING TO 5000'. THE F/O WENT TO THE FAA AND HEARD THE TAPE OF THE INCIDENT AND FOUND THAT HE WAS GIVEN AN ALT OF 8000' AND ACKNOWLEDGED FOR THAT ALT. THE ALT OF 5000' WAS NEVER SPOKEN. RPTR INSISTED THAT THE CTLR'S CONFUSING CLRNC USING A COMBINATION OF HIGH AND LOW ALT STRUCTURE CAUSED THE F/O TO HEAR AND SET THE WRONG ALT. BOTH PLTS WERE USING CABIN SPEAKERS AND DID NOT HEAR EVERYTHING CLEARLY.