FLC OF AN LGT OVERSHOT CRUISE ALT DURING CLB DUE TO 'PF' DISTR AND A LOW VOLUME ALT ALERTER.

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ABOUT 1000 FT PRIOR TO LEVELING OFF AT OUR ASSIGNED ALT OF FL310; I GAVE THE CAPT (PNF) AND THE COPLT (PF) THE PERFORMANCE NUMBERS (EPR AND AVAILABLE GS) FOR CRUISE. THE CAPT HAD CALLED OUT 'TWO TO GO' AT FL290 AND THE COPLT (PF) REPEATED 'TWO TO GO.' AS HE PUT THE EPR NUMBERS IN THAT I GAVE HIM; THE CAPT QUESTIONED THE NUMBERS; THINKING THEY WERE TOO LOW. SO I GOT THE PERFORMANCE MANUAL OUT AND RECHKED MY NUMBERS. THEY WERE CORRECT AND THE CAPT AND COPLT AGREED WHEN THEY SAW THE NUMBERS FOR THEMSELVES. I TURNED TO PUT THE BOOK AWAY AND STARTED TO ENTER TKOF TIMES IN THE ACFT LOGBOOK; WHERE I HEARD THE CAPT SAY SOMETHING TO THE EFFECT THAT WE WERE TOO HIGH. AS I LOOKED; I SAW OUR ALTIMETERS READ FL315. FL310 WAS SET IN OUR ALTIMETER WARNING INDICATOR. THE COPLT IMMEDIATELY STARTED A DSCNT BACK TO FL310. CTR ASKED US TO CONFIRM THAT WE WERE ASSIGNED FL310. (THE CTLR ASKED; 'SAY YOUR ASSIGNED ALT.') THE CAPT STATED THAT WE WERE ASSIGNED FL310 AND THAT WE WERE AT FL312 AT THAT MOMENT AND THAT WE HAD HAD A PROB. OUR ALT WARNING INDICATOR WORKED; BUT THE TONE THAT (AURAL TONE) IT EMITTED WAS MUCH WEAKER THAN NORMAL. I FEEL THE PF GOT SIDETRACKED FROM HIS PRIMARY RESPONSIBILITY OF FLYING THE ACFT AT A PARTICULARLY CRUCIAL TIME (NEAR LEVELOFF) TO GET INVOLVED IN A VERY INSIGNIFICANT ITEM OF WHETHER THE CRUISE EPR NUMBERS WERE CORRECT OR NOT. THIS WOULD NOT HAVE OCCURRED HAD EVERYONE WAITED UNTIL WE WERE LEVELED OFF FIRST. THERE WERE NO OTHER ACFT NEAR US AT THE TIME AND NO EVASIVE ACTION WAS NECESSARY.