A319 CREW WAS ISSUED A GAR BY THE ATCT LCL CTLR; BECAUSE THERE WAS AN ACFT STILL ON THE RWY.

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UNEXPECTED FOG CAUSING CAT IIIB APCHS TO DEN; CLR ABOVE THE FOG LAYER. FOG EXTENDED FROM 500 FT TO THE SURFACE. DEN APCH TOLD US TO MAINTAIN 170 KTS UNTIL THE MARKER. WINDS ALOFT OVER THE MARKER WERE 26 KTS; QUARTERING TAILWIND. 1-2 MI PRIOR; WE GOT SPD RELIEF. SURFACE WINDS WERE 260 DEGS/6 KTS. AT APPROX 400 FT; THE TWR DIRECTED US TO GO AROUND DUE TO AN ACFT ON THE RWY. I AS FO; PNF; ATTEMPTED TO REPLY BUT WAS UNABLE. NOT SURE IF IT WAS DUE TO MIKE FAILURE; OPERATOR FAILURE OR BEING COVERED BY THE TWR CTLR. AFTER MY 2ND ATTEMPT AND REPEATED NEAR FRANTIC CALLS FROM THE TWR; THE CAPT WAS ABLE TO COMMUNICATE WITH THE TWR THAT WE WERE EXECUTING A MISSED APCH. SOMETIME DURING THE GAR; THE AUTOPLT DISENGAGED. AT APPROX 350-300 FT; THE CAPT POSITIVELY DISCONNECTED THE AUTOPLT WITH THE DISCONNECT BUTTON AND ROTATED THE ACFT TO GO AROUND GUIDANCE. WE RECEIVED 2 'TOO LOW TERRAIN' AUDIO MESSAGES DURING THE MANEUVER. WE CLBED TO 9000 FT; ENTERED DOWNWIND AND SUCCESSFULLY LANDED ON RWY 35R. SUPPLEMENTAL INFO FROM ACN 613121: AT ABOUT 400 FT RADIO ALTIMETER; TWR CTLR BROKE THE SILENCE WITH AN URGENT; VERY STARTLING; 'ACR X; GO AROUND!' I EXECUTED THE GAR MANEUVER AS THE FO REPLIED TO TWR. ALTHOUGH I COULD HEAR THE FO'S REPLY IN THE COCKPIT; I REALIZED THAT HIS XMISSION DID NOT GO OUT. THE TWR CTLR REPEATED; WITH MORE ANXIETY IN HIS VOICE; 'ACR GO AROUND! ACKNOWLEDGE!' THE FO REPLIED AGAIN; BUT AGAIN; HIS XMISSION DID NOT GO OUT. I REPLIED TO THE CTLR; 'ACR X; GOING AROUND.' AT ABOUT THIS TIME; THE CABIN TO COCKPIT INTERPHONE LIGHT WAS FLASHING (THE CHIME HAD BEEN INHIBITED). THE AIRPLANE WAS SAFELY FLYING; BUT WAS NOT PERFORMING PROPERLY. THE AUTOPLT HAD AT SOME POINT BECOME DISENGAGED. I ROTATED TO GUIDANCE AND MANUALLY FLEW MISSED APCH. AS I DID SO; 2 'TOO LOW TERRAIN' GPWS ALERTS SOUNDED. THE REASON FOR THE CABIN CALL TO COCKPIT WAS THAT THE PURSER 'JUST WANTED TO KNOW WHAT WAS GOING ON.'