MLG IN THE HOLD DID NOT DESCEND AS CLEARED.

Narrative

WE WERE HOLDING AT THE GRACE INTXN AT 10000 AND GIVEN AN EFC TIME OF ABOUT 30 MINUTES. THERE WERE OTHER ACFT HOLDING IN 1000' INCREMENTS ABOVE US AND ONE ACFT 1000' BELOW US. I WAS ON THE PA TELLING THE PAX ABOUT OUR HOLDING DELAY AND EXPECTED ARRIVAL TIME AT THE GATE. THE ACFT WAS ON AUTOPLT AND WAS COUPLED TO THE FMC TO KEEP US IN THE HOLD AT GRACE. THE CAPT RECEIVED A DSCNT CLRNC TO 9000' AND RESET THE ALT WINDOW IN THE AUTOPLT. I DID NOT NOTICE THIS AND DID NOT HEAR THE CLRNC SINCE I WAS TALKING ON THE PA. SHORTLY AFTER I HAD FINISHED TALKING; APCH CTL CALLED AND ASKED IF WE HAD STARTED OUR DSCNT YET; AND THEN CALLED OUT A TFC ADVISORY. THE CAPT REMARKED THAT WE HAD BEEN CLRD TO 9000' AND I IMMEDIATELY DESCENDED TO THAT ALT. I DON'T KNOW WHETHER OUR DELAYED DSCNT CAUSED A TFC CONFLICT. THE CTLR DID NOT SAY SO. MOST CAPTS I HAVE FLOWN WITH HAVE WHOEVER IS FLYING THAT PARTICULAR LEG; MAKE ALL THE PAX ANNOUNCEMENTS. YOU CANNOT MONITOR THE ATC RADIO WHILE DOING THIS; SO CONSEQUENTLY THE PERSON FLYING THE ACFT IS TEMPORARILY OUT OF THE COMMUNICATIONS LOOP. THE PLT NOT FLYING DOESN'T USUALLY TAKE OVER CONTROL OF THE ACFT WHILE THE OTHER PLT IS ON THE PA; SO YOU HAVE TO REMEMBER TO UPDATE THE PLT FLYING ON ANY CHANGES WHILE HE WAS OFF THE RADIO. PERHAPS A BETTER TECHNIQUE WOULD BE TO HAVE THE PLT FLYING MONITOR THE ATC FREQ AND THE PLT NOT FLYING TO MAKE THE PAX ANNOUNCEMENTS.