2 MAN CREW; ON DSCNT; OVERSHOT ASSIGNED ALT.

Narrative

CAPT WAS FLYING. I WAS WORKING THE RADIOS. WE WERE AT 11000 FT MSL LAST ASSIGNED ALT JUST ENDING THE STAR INTO STL. WE WERE STILL WITH CTR ATC. I INFORMED THE CAPT I NEEDED TO LEAVE THE ATC FREQ TO MAKE THE COMPANY IN-RANGE CALL AND CALL COMPANY MAINT FOR A MINOR WRITE-UP. THE CAPT WAS NOW FLYING AND HANDLING THE ATC FREQ. I WAS OFF TALKING TO COMPANY FOR SEVERAL MINS. WHEN I RETURNED TO LET THE CAPT KNOW I WAS BACK TO HANDLE ATC COMS AGAIN; I NOTICED WE MUST HAVE BEEN HANDED OFF TO APCH BECAUSE HE SWITCHED FREQS AND HE CHKED ON WITH APCH. DURING ALL THIS; I HAD THE VOLUME ON MY #1 RADIO TURNED DOWN (ATC RADIO). SO WHEN I RETURNED; I DID NOT HEAR HIS EXCHANGE WITH APCH. THEY GAVE US A LOWER ALT BECAUSE HE DIALED 7000 FT MSL INTO THE ALT WINDOW. AFTER THAT; I WAS BACK ON #1 (ATC) RADIO; ASSUMING WE WERE CLRED TO 7000 FT. WHEN WE WERE ALMOST TO 7500 FT; THE CAPT ASKED ME TO VERIFY WITH ATC THAT WE WERE CLRED TO 7000 FT. I DID AND ATC SAID 'NO;' WE WERE ONLY CLRED TO 8000 FT. THE CAPT THEN IMMEDIATELY CLBED BACK TO 8000 FT AND NOTHING MORE WAS SAID BY ATC. THE REST OF THE FLT WAS UNEVENTFUL. NEXT TIME I'LL TRY TO MAKE COMPANY CALLS EARLIER BEFORE APCH DUTIES BECOME VERY BUSY. I WILL ALSO KEEP VOLUME UP SLIGHTLY SO I CAN MONITOR BOTH FREQS.