A B737-300 PIC FAILS TO FOLLOW THE ARR PROC AS AMENDED BY LAX APCH CTL INTO LAX; CA.

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WE WERE FLT TUS-LAX. OUR LNAV WAS PROGRAMMED FOR THE PDZ 3 ARR TO INTERCEPT THE RWY 25L LOC AT ARNES. THIS IS USUALLY THE PROPER ARR INTO LAX FROM THE TUCSON AREA. AS CAPT; I WAS FLYING THE AIRPLANE; AND THE FO WAS HANDLING THE RADIOS. SOMEWHERE PRIOR TO ARNES WE WERE GIVEN A NEW CLRNC WHICH WAS TO INTERCEPT THE RWY 24R LOC AT MITTS. I NEVER HEARD THIS XMISSION AND THE FO ACKNOWLEDGED THE CLRNC; BUT DID NOT UPDATE THE LNAV FOR THE NEW CLRNC. AT ARNES THE ACFT INTERCEPTED THE RWY 25L LOC. THE FO ASKED ME WHERE I WAS GOING AND I INDICATED I WAS INTERCEPTING RWY 25 AS PER OUR CLRNC. HE INFORMED ME THAT THE CLRNC HAD CHANGED AND HE THOUGHT I KNEW THAT. WE BEGAN TURNING BACK TO THE ARR WHEN THE CTLR INQUIRED WHAT WE WERE DOING. WE TOLD HIM THAT WE HAD MADE A MISTAKE AND WERE TRYING TO CORRECT IT. VECTORS WERE GIVEN TO RWY 24R LOC AND THE REST OF THE FLT WENT AS PLANNED. OUR COM ON THE FLT DECK BROKE DOWN WHICH CAUSED THIS MISTAKE. I DON'T KNOW WHY I DIDN'T HEAR THE NEW CLRNC AND I DON'T KNOW WHY THE LNAV WAS NOT UPDATED. SUPPLEMENTAL INFO FROM ACN 436266: WE WERE CLRED THE ARR AND INTERCEPT THE RWY 24R LOC. THE LNAV WAS NOT CHANGED FOR THE APPROPRIATE RWY AND BEGAN TO TURN TO INTERCEPT RWY 25L. AFTER ABOUT 70 DEGS OF TURN WE REALIZED IT WAS TURNING TOO SOON; WE BEGAN TO TURN BACK TOWARDS THE PDZ ARR WHILE AT THE SAME TIME APCH SAYS; 'IT LOOKS LIKE YOU'RE INTERCEPTING RWY 25L' AND THEN GAVE US A VECTOR TOWARDS RWY 24R.