SMA HAS NMAC WITH SECOND SMA DUE TO BUMPING AUDIO SWITCH AND CUTTING OFF COM.

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ON APCH TO LVK MY INST STUDENT WAS SETTING UP AND IDENTING HIS NAV RADIOS FOR APCH. I WAS REVIEWING THE APCH AND MONITORING THE STUDENT; BUT I FAILED TO NOTICE THAT HE HAD REPOSITIONED THE SWITCHES ON THE AUDIO PANEL IN SUCH A WAY SO AS TO CUT OUT THE COM RADIO WE WERE USING TO RECEIVE STOCKTON APCH. THE FREQ HAD BEEN FAIRLY BUSY; AND AFTER A COUPLE MINS OF SILENCE I NOTICED THE AUDIO PANEL SWITCHES HAD BEEN MOVED TO THE WRONG POS. ABOUT THE SAME TIME I NOTICED TFC APPROX 200 FT ABOVE US; BUT OTHERWISE ON A COLLISION COURSE. IMMEDIATELY AFTER REPOSITIONING ITS SWITCHES WE RECEIVED A TFC ALERT FROM APCH; (WHO HAD TRIED SEVERAL TIMES TO CONTACT US) AND WERE ADVISED TO IMMEDIATELY CLB 500 FT. WE RPTED TFC IN SIGHT AND REMAINED AT 3300 FT. A CLB TO 3800 FT WOULD PROBABLY HAVE RESULTED IN A COLLISION; SINCE THE CONFLICTING TFC PASSED A SCANT 200 FT ABOVE US. (APPEARED TO BE A CESSNA 150). AT THE TIME I HAD MY STUDENT BOTH FLYING THE AIRPLANE AND SETTING UP FOR THE APCH; WITHOUT ANY VIEW RESTRICTING DEVICES; IN AN ATTEMPT TO SIMULATE SINGLE PLT IFR IN VMC CONDITIONS. IN THIS CASE THE STUDENT'S HANDS WERE QUICKER THAN THE INSTRUCTOR'S EYES; AND THE SMALL SWITCHES WEREN'T OBVIOUS ENOUGH TO IMMEDIATELY NOTICE. AFTER ASKING SEVERAL QUESTIONS ABOUT THE SWITCHES; IT BECAME OBVIOUS THAT THE STUDENT KNEW HOW TO PROPERLY WORK THE AUDIO PANEL; BUT THE SMALL TOGGLE TYPE SWITCHES USED WERE EASILY ACTUATED WITH VERY LITTLE PRESSURE. APPARENTLY HE UNINTENTIONALLY MOVED THE SWITCH OUT OF THE AUTO POS AS HE WAS REMOVING HIS HAND FROM THE AUDIO PANEL.