PLT OF AN SMA SEL LOST RADIO COM WITH ATC FOR ADVISORY SVC DUE TO INADVERTENTLY TURNING OFF THE AUDIO SWITCH FOR THE COM RADIOS.

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I DEPARTED IAD TOWARD W66 AT XA10 PM. I WAS AT 2500 FT UNDER CTL OF IAD APCH INSIDE IAD CLASS B AIRSPACE. (AFTER 15 DME I WAS NO LONGER IN CLASS B; BUT UNDER THE OUTER RING.) SHORTLY BEFORE 20 DME FROM AML I NOTED A BIZJET PASS DIRECTLY OVERHEAD FROM 2 O'CLOCK AT ABOUT 500 FT ABOVE. AT ABOUT 20 DME FROM AML VOR I TRIED TO REQUEST FREQ CHANGE FROM IAD APCH; AS I WAS OUTSIDE CLASS B; AND WANTED TO CONTACT W66 TFC ADVISORY FREQ. I COULDN'T RAISE IAD APCH. I TRIED THE OTHER RADIO; AND REALIZED NEITHER WAS 'WORKING.' BOTH WERE SILENT. USING MY FLASHLIGHT ON THE AUDIO PANEL I DISCOVERED I HAD INADVERTENTLY SWITCHED THE 'AUTO' SWITCH TO THE NEUTRAL POS; SO THAT I COULD HEAR NOTHING ON SPEAKER OR PHONES. SWITCHING IT TO PHONE; IMMEDIATELY THE IAD APCH FREQ BECAME ALIVE AGAIN. BEFORE I COULD CALL TO REQUEST FREQ CHANGE; IAD APCH CALLED ME; ASKED IF I HAD HEARD PREVIOUS XMISSIONS. I ANSWERED 'NO' AND VOLUNTEERED THAT I HAD SET A SWITCH WRONG. HE SAID HE HAD TRIED TO CALL SEVERAL TIMES INCLUDING TO CALL OUT THE 'TFC' I 'HAD FLOWN THROUGH.' HE DIDN'T KNOW IF IT WAS THE BIZJET I HAD SEEN BECAUSE THE TFC WAS SQUAWKING VFR. MY MISTAKES WERE BELIEVING THAT THE 'QUIET' APCH CTL FREQ WAS NORMAL; AND NOT USING THE FLASHLIGHT EARLIER. (THE AUDIO PANEL IS UNLIGHTED.) FROM NOW ON IN THAT ACFT I WILL USE THE FLASHLIGHT WHENEVER I USE THE AUDIO PANEL AT NIGHT OR IF THE RADIOS GO QUIET AT NIGHT.