What was found
AIRCRAFT WAS NOT GROUNDED: UNCOMMANDED EVACUATION ALARM SOUNDED AFTER TAKEOFF. ALARM WENT AWAY WHEN TOGGLED INTO THE "CAPTAIN" POSITION. FLIGHT ATTENDANT STATED THAT BEFORE ALARM WENT OFF THAT THERE WAS CRACKLING THROUGH THE PA SYSTEM. THAT NOISE IS STILL PERSISTING. ALSO, THE SEAT BELT SIGN AND MULTIPLE OTHER LIGHTS WERE FLASHING IN THE CABIN BEFORE AND DURING THE EVACUATION ALARM. MAINTENANCE REPLACED (DEU) DECODER ENCODER UNIT B 300RH5 AND ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK PER AMM 23-73-47.