AN MD80 EXPERIENCED ATC COM DIFFICULTY WHEN AN INTERNAL FAILURE OF THE OBSERVER'S AUDIO PANEL MADE THE OBSERVER'S OXYGEN MASK MIKE HOT.

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WE DEPARTED ON THIS FLT AND ALL COMS WERE INITIALLY NORMAL. AT CRUISE ALT WHILE ON ZFW FREQ; WE STARTED TO HAVE COM PROBS. AT FIRST WE HAD A DIFFICULT TIME CHKING IN. TRIED A DIFFERENT RADIO AND WE WERE ABLE TO REESTABLISH COMS FOR A SHORT WHILE. THEN THE PROBS RESTARTED. WE HAD INTERMITTENT COMS WITH ATC. THE CTLR SAID HE COULD HEAR US; BUT THAT WE APPEARED TO HAVE A HOT MIKE AND WERE BLOCKING HIS FREQ. NEEDLESS TO SAY; THE CTLR WAS IRRITATED AND THAT IS UNDERSTANDABLE. WE THEN SWITCHED FREQS AND HAD A DIFFICULT TIME CHKING IN. AFTER HAVING DIFFICULTIES ON THE SECOND FREQ; IT APPEARED FAIRLY OBVIOUS THAT THE PROB WAS WITH US AND THAT SWITCHING RADIOS DID NOT SOLVE THE PROB. WE TROUBLESHOT THE COM SYS AND UNPLUGGED VARIOUS MIKES; ETC; MADE SURE NO BUTTONS WERE PUSHED ON THE OBSERVER'S PANEL AND THOUGHT WE HAD THE PROB ISOLATED TO THE FO'S AUDIO PANEL. AT THAT POINT WE SWITCHED ROLES; WITH THE FO TAKING THE REMAINDER OF THE LEG AND I AS CAPT TOOK OVER RADIO COMS AS MY PANEL SEEMED TO WORK JUST FINE. WE NOTIFIED MAINT WHO MET THE ACFT AND DETERMINED THAT THE PROB ACTUALLY APPEARED TO INVOLVE A STUCK MIKE ON THE OBSERVER'S OXYGEN MASK MIKE. THE PROB WAS INTERNAL TO THE OBSERVER'S AUDIO PANEL. HE THEN REPLACED THE OBSERVER'S AUDIO PANEL AND WE FLEW THE ACFT FOR 2 MORE LEGS WITH NO FURTHER PROBS WITH OUR COMS. THIS WAS A VERY UNCOMFORTABLE SITUATION FOR ALL INVOLVED AS COM PROBS INFLT CAN BE QUITE DANGEROUS.