A B757-200 ON APCH AT 4000 FT DECLARED AN EMER AND DIVERTED DUE TO SMOKE IN THE CABIN CAUSED BY AN OVERHEATED SEAT AUDIO CTL BOX.

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ACR FLT XYZ ON APCH TO LGA. WE WERE CLRED FOR EXPRESSWAY VISUAL RWY 31 WHEN A FLT ATTENDANT CALLED COCKPIT ON INTERPHONE AND RPTED 'FIRE IN THE CABIN.' I QUERIED ABOUT VISIBLE FLAMES AND SHE RPTED NONE. I DECLARED EMER; SQUAWKED 7700; AND REQUESTED STRAIGHT-IN LNDG RWY 4 WITH EQUIP STANDING BY. A SHORT TIME LATER; WE NOTICED A 'PACK INOP' LIGHT AND A 'PACK TEMP' EICAS MESSAGE. THE FO WAS FLYING AND I DIRECTED HIM TO CONTINUE WHILE I EXECUTED CHKLISTS AND COMMUNICATED WITH THE LEAD FLT ATTENDANT. SINCE OUR ALT WAS SO LOW; I RAISED THE CABIN ALT AND MANUALLY OPENED THE OUTFLOW VALVE TO AID IN SMOKE REMOVAL. ALSO; THERE WAS NO SMOKE IN THE FORWARD PART OF CABIN OR COCKPIT; SO OUR PROX TO THE ARPT AND GND; PLUS THE URGENCY OF THE SIT; PRECLUDED THE OPTION OF DELAYING THE LNDG TO DON MASKS AND GOGGLES OR RUN DETAILED CHKLISTS. WITH A FIRE RPTED ONBOARD; GETTING SAFELY ON THE GND IS TOP PRIORITY. WE LANDED UNEVENTFULLY ON RWY 4. WE TAXIED CLR AND I DIRECTED THE FO TO ANNOUNCE 'REMAIN SEATED' WHILE I PARKED ON TXWY R; N OF RWY 13/31. I AGAIN COMMUNICATED WITH THE FLT ATTENDANTS AND WAS ABLE TO DISCERN THAT THERE WERE NO FLAMES OR BREATHING PROBS BUT SOME PAX WANTED TO EVAC. I DIRECTED THE FO TO GO BACK AND ASSESS CONDITIONS WHILE I REQUESTED AN ARFF FREQ AND DETERMINED THAT THERE WAS NO SIGN OF FIRE OUTSIDE. I AGAIN TOLD PAX TO STAY CALM AND SEATED. THE FO RPTED THAT THERE WAS A STRONG ELECTRICAL SMELL BUT SMOKE SEEMED TO HAVE DISSIPATED. I THEN DECIDED THAT THE THREAT OF INJURIES DURING AN UNNECESSARY EVAC OUTWEIGHED THE DANGER OF STAYING ONBOARD NOW THAT THE SIT HAD STABILIZED. I TOLD ARFF; ATC; AND PAX THAT WE WOULD TAXI TO THE GATE FOR A NORMAL DISEMBARKATION. I REQUESTED THE EQUIP FOLLOW US TO THE GATE. AT THE GATE; I AGAIN TOLD PAX TO DISEMBARK NORMALLY THROUGH THE JETWAY. MAINT DISCOVERED A HOT; SMOKING AUDIO ELECTRONIC BOX BELOW A SEAT. ACCORDING TO PAX AND FLT ATTENDANT RPTS; THE SMOKE APPEARED TO BE ENTERING FLOOR VENTS. PACK TEMP PROB MAY HAVE BEEN COINCIDENTAL OR SOMEHOW RELATED TO THE SMOKE ENTERING FLOOR VENTS. THE AUDIO BOX WAS COMPLETELY SHROUDED IN METAL; PREVENTING THE FIRE FROM SPREADING. BUT NOTHING REPLACES A PERSONAL ASSESSMENT BY A COCKPIT CREW MEMBER. OF COURSE; IN A 2 PLT COCKPIT; THIS IS ONLY POSSIBLE ON THE GND.