B757 ALTDEV IN HOLDING PATTERN ON ARR TO ATL.

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UPON ARR IN ATLANTA; WE WERE GIVEN HOLDING AT MACEY INTXN. WHEN ENTERING HOLDING; WE WERE DSNDING TO ASSIGNED FL250; PASSING APPROX FL260. AFTER THE OUTBOUND TURN IN HOLDING; I INFORMED CAPT THAT I WANTED TO MAKE A PA TO THE PAX CONCERNING THE DELAY/HOLDING. IT WAS THE CAPT'S LEG AND I HAD BEEN TALKING TO ATC SO HE TOOK THE ATC RADIO WHILE I MADE THE PA. WHILE TALKING ON THE PA; I SAW THE CAPT RESET THE ALT CTLR TO FL230. MY ATC VOLUME WAS DOWN SO I DIDN'T HEAR THE INSTRUCTIONS OR THE CAPT'S READBACK. AFTER THE PA; AND DURING THE INBOUND TURN TO MACEY INTXN; WE WERE DSNDING THROUGH FL236 AND ZTL SAID TO VERIFY WE WERE LEVELING AT FL240. I REPLIED WE WERE DSNDING TO FL230. THE CTLR INSTRUCTED US TO RETURN TO FL240. THE CAPT SAID HE HAD HEARD AND READ BACK 'FL230.' WE RETURNED TO FL240 AND NOTHING FURTHER WAS SAID ABOUT THE DEV BY ATC. WHILE MY PA TO THE PAX ONLY TOOK ABOUT 1 MIN; I SHOULD HAVE HAD THE ATC RADIO VOLUME AT LEAST PARTIALLY UP. I TURNED IT DOWN BECAUSE OF THE HIGH VOLUME OF RADIO TFC. I DIDN'T QUESTION THE ALT CHANGE BECAUSE WE HAD BEEN STEPPED DOWN 1000-2000 FT AT A TIME FROM CRUISE ALT; AND IT SEEMED 'NORMAL' TO ME.