B737-400 CREW DID NOT COMPLY WITH AN ALT XING RESTR ON THE FUZZY 4 INTO LAS.

Narrative

OUR ACFT WAS IN A VERT SPD DSCNT (1500 FPM) FROM FL310 TO FL240 WHEN SUBSEQUENTLY CLRED TO DSND TO 16000 FT AT FUZZY ON THE FUZZY 4 ARR INTO LAS. AT THE TIME OF THE CALL; THE CAPT WAS INVOLVED IN A TECHNICAL CONVERSATION WITH 2 JUMP SEAT PLTS IN OUR COCKPIT; AND WE ALSO RECEIVED A CALL FROM THE CABIN LEAD FLT ATTENDANT ABOUT AN IRATE PAX IN FIRST CLASS COMPLAINING ABOUT THE POOR JOB THE PLTS WERE DOING AVOIDING AND KEEPING THE PAX INFORMED ABOUT INFLT TURB CONDITIONS. I; THE PF; ANSWERED THAT CALL WHILE THE CAPT RECEIVED THE CONTINUED DSCNT CLRNC. WE HAD BEEN ENCOUNTERING LIGHT TO MODERATE TURB FOR ABOUT 40 MINS AND HAD CHANGED ALTS 4 TIMES LOOKING FOR SMOOTH AIR. I DID NOT HEAR OR ACKNOWLEDGE THE NEW CLRNC; THOUGH THE CAPT DID ENTER THE ALT IN THE MCP; AND I ALREADY HAD PREBUILT IT INTO THE FMC SINCE I WAS EXPECTING IT. DISTR FROM MY PRIMARY DUTY OF FLYING THE ACFT; I FAILED TO ENGAGE VNAV PATH FUNCTIONS OR INCREASE THE CTLED DSCNT RATE TO MEET THE RESTR. I NOTED THE ERROR 2 MI PRIOR TO FUZZY AS I WAS PASSING THROUGH FL190 IN DSCNT. ATC NEVER QUERIED US. OUR ERROR AS A CREW WAS IN ALLOWING A CABIN SIT TO DISTRACT US IN A DSCNT AND A PROCEDURAL ERROR BY NOT CONFIRMING AN ALT CLRNC BTWN PLTS. THE FLT ATTENDANT SHOULD NOT HAVE DISTURBED US DURING DSCNT UNLESS IT WAS A SERIOUS SIT. NO TFC CONFLICTS RESULTED.