MD80 PA SYSTEM BROADCASTS AN 'ALARM' SOUND EACH TIME IT IS USED. UNABLE TO IDENTIFY THE SOURCE OF THE ALARM; THE FLT CREW ELECTS TO RETURN TO DEP ARPT.

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I WAS CAPT ON FLT FROM ZZZ. THE FO WAS FLYING WHILE I WAS PNF. THE TIME OFF THE GND WAS XA03Z. AFTER CLBING THROUGH 10000 FT; THE FLT ATTENDANT CALLED THE COCKPIT AND TOLD ME OF AN ALARM THAT WAS SOUNDING IN THE CABIN. I ASKED IF IT WAS THE LAVATORY SMOKE ALARM; SHE SAID THEY WERE NOT SURE WHERE THE SOUND WAS COMING FROM OR WHICH ALARM IT WAS. EVERY TIME SHE TRIED TO MAKE A PA ANNOUNCEMENT THIS ALARM WOULD SOUND. I HAD THEM TRY THE AFT CABIN PA; AND THE ALARM SOUNDED THEN AS WELL. BY THIS TIME THE FLT WAS CLBING THROUGH 20000 FT; WHILE I TRIED TO DIAGNOSE THE PROB. THERE WAS NO EVIDENCE OF SMOKE OR FIRE; EITHER IN THE CABIN OR COCKPIT INDICATIONS; AS I RULED THIS OUT IMMEDIATELY. I CONTACTED MAINT CTL; AND TOLD THEM THE SITUATION. THEY SAID UH; WE THINK YOU SHOULD COME BACK. I ASKED; WHAT ELSE COULD THIS BE? WHAT ALARM COULD THEY BE TALKING ABOUT? AND DO YOU HAVE ANY OTHER IDEAS? THE RESPONSE WAS AGAIN; IF IT'S STILL GOING OFF; THEN COME BACK. WE COORDINATED AN AIR RETURN WITH ATC; AND ZZZ OPS; ETC. THE TIME BACK ON THE GND WAS XB27Z; 1 HR 24 MIN FLT TIME. AFTER LNDG WHILE TAXIING TO THE GATE; A PAX STOOD UP AND WENT INTO THE LAVATORY. I HEARD THE FLT ATTENDANT SHOUTING SIT DOWN; SIT DOWN; I BROUGHT THE ACFT TO A STOP. THE FLT ATTENDANT CALLED THE COCKPIT AND SAID SOMEONE WAS IN THE LAVATORY. I HEARD MORE SHOUTING FROM CABIN; BUT COULD NOT REALLY HEAR EXACTLY WHAT WAS SAID. AT THIS TIME I ASSUMED THE PAX; WHO CAME OUT OF THE LAVATORY WAS ARGUING WITH MY FLT ATTENDANTS. AS A PRECAUTION; I ASK THE FO TO CALL OPS TO HAVE POLICE STANDING BY AT THE GATE; IN CASE THIS WAS A PAX DISTURBANCE. AFTER WE BLOCKED INTO THE GATE; AND OPENED THE COCKPIT DOOR; I WAS INFORMED THE SHOUTING I HEARD WAS OUR FLT ATTENDANT YELLING INFO TO THE PAX; BECAUSE SHE STILL THOUGHT THE PA WAS MALFUNCTIONING AND UNUSABLE AS IT WAS INFLT. AFTER EVERYONE DEPLANED; AND I TALKED WITH MAINT TO TRY TO TROUBLESHOOT THE PROB FURTHER. IT WAS SUGGESTED AS A POSSIBILITY THAT THE IGNITION WAS SET TO OVERRIDE ACCIDENTALLY (INSTEAD OF OFF); AND SOUND OF THE IGNITERS FIRING WAS BEING PICKED UP BY THE PA SYS; AND AMPLIFIED THROUGHOUT THE CABIN. THUS; SOUNDING LIKE THE ALARM THAT WAS BEING REPORTED. I CANNOT SAY WITH 100% CERTAINTY THAT THIS WAS THE CASE. HOWEVER; IT IS A PLAUSIBLE EXPLANATION. DURING FINAL APCH ON THE AIR RETURN; DURING THE LNDG CHKLIST; I DID NOTICE THE IGNITION SWITCH FEEL A LITTLE DIFFERENT THAN USUAL (NORMAL FEEL TO GO FROM OFF TO ON IS 1 DETENT). THIS TIME; IT FELT LIKE 1 PLUS A LITTLE MORE DETENTS; PERHAPS 1.5 DETENTS. I THOUGHT THIS WAS STRANGE; BUT THE PRIORITY AT THIS TIME WAS LNDG THE AIRPLANE. WE SWAPPED AIRPLANES AND COMPLETED THE ROUND TRIP UNEVENTFULLY. SUGGESTED CORRECTIVE ACTIONS: THIS HAS DEFINITELY BEEN A LEARNING EXPERIENCE FOR ME; AND THIS WILL NEVER HAPPEN TO ME AGAIN. I WOULD LIKE TO SEE IN THE FUTURE ADDITIONAL THOUGHTS AND INPUT FROM MAINT CTL WHILE PROVIDING ASSISTANCE.