SMA ON IFR FLT PLAN TEMPORARILY LOST COM DUE TO USING WRONG SWITCH.

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APCHING LIT; MONITORING APCH CTLR FREQ. SUDDENLY REALIZED AUDIO HAD CEASED ON BOTH COM RECEIVERS. CHKED FREQS; SWITCHED HEADSET AND TRIED R SEAT MIKE/HEADSET JACKS TO NO AVAIL. SW OF LIT; IMC CONDITION EXISTED AT MY ALT. IDENTED TWICE; 30 SECONDS APART ON ACQUIRED ATC CODE; WAITED 15 SECONDS. CHANGED TO XPONDER CODE 1200. CAREFULLY CHKED VISUALLY FOR TFC AND BEGAN DSNDING L TURN TO MAINTAIN FLT IN VFR CONDITIONS. REMAINED ABOVE AND OUTSIDE OF CLASS 'C' AIRSPACE. FOUND ALL OTHER AVIONIC GEAR TO BE OPERATING NORMALLY. RECHK OF AUDIO SELECTOR PANEL REVEALED THAT FOLLOWING TUNING AND IDENTING LIT RWY 4 LOM ON ADF. I HAD INADVERTENTLY DESELECTED THE 'AUTO' FUNCTION OF THE PANEL LOCATED NEXT TO ADF AUDIO SELECTOR INSTEAD OF ADF SELECTOR BUTTON. (ADJACENT SWITCHES; SAME SIZE AND SHAPE.) CORRECTED SWITCH POS AND RE-ESTABLISHED CONTACT WITH LIT APCH CTL. EXPLAINED BRIEFLY; CTLR SAID NO PROB; I REQUESTED CLRNC OVER RTE FILED; WAS CLRED AS FILED AT ALT REQUESTED (4000 FT MSL). ON THIS AUDIO PANEL; AUTO SWITCH MATCHES RECEIVER TO SELECTED XMITTER (#1 OR #2) WITH AUTO OFF AND NO INDIVIDUAL AUDIO SELECTION ACCOMPLISHED; NO AUDIO IS RECEIVED. IF THIS SWITCH WERE DIFFERENT SIZE OR SHAPE; OR A TOGGLE TYPE; I WOULD AID IDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THERE IS NO EASY FIX WITHOUT REWIRING THE AUDIO UNIT. THIS IS A STANDARD RADIO AUDIO SELECTOR PANEL AND HE BELIEVES THE MANUFACTURER HAS BEEN MADE AWARE OF THE PROB AND WILL ADAPT FOR FUTURE UNITS.