What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED ON ARRIVAL TO AUS DUE TO SMOKE IN CABIN, NEAR SEATS 21A, 21B, 21C. SMOKE FIRST APPEARED UNDER THE SEATS. NOTE: VIDEO SCREENS AT 21A, 21B, 21C ARE NOW INOPERATIVE. THE AIRCRAFT LANDED SAFELY AT AUS AND TAXIED TO THE GATE WITHOUT FURTHER INCIDENT. MAINTENANCE FOUND A STRONG BURNT SMELL FRO THE QUAD SEAT ELECTRONIC BOX AT 20ABC AND REPLACED IT PER AMM 44-21-00. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED.