A B712 CREW; ARRIVING PIT; PLANNED AN APCH TO THEIR EXPECTED RWY; INSTEAD OF THEIR ASSIGNED RWY.

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APCHED FIELD ON VECTOR AT 5000 FT. GIVEN EASTERLY HDG UNTIL ABOUT 4-5 MI E OF FIELD. HAD BEEN TOLD BY INITIAL APCH FREQ TO EXPECT VISUAL TO RWY 32. OVER FIELD; FREQ CHANGE MADE; AND WHEN E OF FIELD; A TURN TO S AND DSCNT TO 3000 FT GIVEN. WHEN FIELD CALLED IN SIGHT; GIVEN A HDG OF 250 DEGS AND CLRED VISUAL RWY 28R. I WAS THE CAPT AND FLYING THE ACFT. I BELIEVE THE LOUD AURAL ALERT FOR ALT CHANGE (4000 FT FOR 3000 FT) OBSCURED THE RWY ASSIGNMENT; AND I DID NOT PICK IT UP. THE FO READ IT BACK; BUT DUE TO BEING NEW ON THE AIRPLANE; AND UNSURE OF THE RELATIONSHIP TO FIELD; PARTICULARLY SINCE WE HAD BEEN GIVEN A HDG OF 250 DEGS SAID NOTHING FOR A WHILE. I HAD VISUAL CONTACT WITH AN ACFT TO THE L; AND STAYED CLR OF HIM. FINALLY THE FO ASKED IF WE WERE GOING TO RWY 28R ABOUT THE TIME APCH ASKED US THE SAME THING. WE WERE GIVEN A HDG CHANGE; AND CLB FOR RESEQUENCING TO THE ARPT. GIVEN A FREQ CHANGE; WE WERE TOLD TO EXPECT RWY 28L; AND SET THE NAV GEAR UP APPROPRIATELY. AFTER BEING VECTORED TO BASE LEG; WE WERE CLRED VISUAL APCH RWY 28R. THIS IS NOT THE FIRST TIME I HAVE BEEN SURPRISED BY THIS APCH CTL. THEY DO NOT ROUTINELY VERIFY ASSIGNED RWY WHEN GIVEN APCH FREQ CHANGE; AND IT IS COMMON TO BE TOLD TO EXPECT RWY 'A' BY ONE FREQ; THEN CHANGE TO ANOTHER CTLR AND BE VECTORED AND CLRED; IN A ROUTINE VOICE; APCH TO RWY 'B.' IT CAN BE VERY CONFUSING. AS ALLUDED TO IN THE NARRATIVE; THE AURAL ALT ALERT AND STABILIZER TRIM ALERT ARE VERY LOUD AND TEND TO BLOCK COM.