A PLT ON A BIENNIAL FLT REVIEW IS SURPRISED TO RECEIVE A FLASHING RED LIGHT FROM THE TWR WHILE ON APCH. AS HE WENT AROUND HE NOTICED THAT THE INSTRUCTOR HAD TURNED THE RADIO VOLUME CTL DOWN SO THAT THE RADIO WOULD NOT INTERFERE WITH HIS INSTRUCTIONS ON FINAL APCH. THE RPTR COMPLAINS THAT THIS IS THE SECOND TIME THAT AN INSTRUCTOR HAS DONE THIS TO HIM.

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AFTER SUCCESSFUL COMPLETION OF THE FLT PHASE OF RPTR'S BFR; THE CFI IN R SEAT OF C170 SUGGESTED AN INTRODUCTION TO INST APCHS; SINCE RPTR IS CONSIDERING INST TRAINING. AFTER A BRIEFING; REVIEW OF THE APCH PLATE; AND EXPLANATION OF THE PROCS ON THE GND; WE TOOK OFF JUST BEFORE SUNSET. CONDITIONS WERE CAVU. RWY 35 WAS IN USE WITH A LIGHT NW WIND. WE WERE TO FLY THE ILS RWY 17 APCH; WITH A MISSED APCH; THEN A SECOND APCH; THEN ENTER THE PATTERN FOR A NORMAL LNDG ON RWY 35. RPTR WAS WEARING A HOOD; CFI WAS SAFETY PLT; AND COACHING RPTR (PIC) THROUGH THE TURNS AND TIMING. INTERCOM BTWN HEADSETS WAS IN USE. AT SOME POINT AFTER OM; CFI TURNED DOWN RADIO VOLUME BECAUSE RADIO TFC BTWN TWR AND OTHER ACFT WAS CONFLICTING WITH HIS INSTRUCTIONS. RPTR REMOVED HOOD AT DECISION HT AND NOTED FLASHING RED LIGHT SIGNAL FROM TWR CAB AND IMMEDIATELY INITIATED MISSED APCH. MY FIRST THOUGHT WAS THAT TWR HAD LOST THEIR RADIO. THEN I LOOKED AT MY OWN NAV COM AND WAS SHOCKED TO SEE VOLUME TURNED DOWN. CFI ADMITTED HE'D FORGOTTEN TO TURN IT BACK UP. I CALLED THE TWR IMMEDIATELY; APOLOGIZED FOR THE LOSS OF COM; AND SET UP FOR THE SECOND APCH. THERE WAS NO DEPARTING TFC (HEAD- ON CONFLICT) AT THE TIME OF THE INCIDENT; BUT AT LEAST ONE OTHER ACFT IN THE PATTERN. THIS IS THE SECOND TIME IN MY SHORT FLYING CAREER THAT A FLT INSTRUCTOR HAS TURNED DOWN RADIO VOLUME WITHOUT MY KNOWLEDGE SO HE CAN TALK WITHOUT 'INTERFERENCE' FROM ATC -- AN EXTREMELY UNSAFE PRACTICE.