PLT OF A SMT WAS DISTR AND FAILED TO HEAR ATC INSTRUCTIONS TO CLB RESULTING IN AN ALT WELL BELOW THE MEA.

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ENRTE MRY TO DPA; FUEL STOP AT 11H. PASSING SAF I BECAME AWARE OF A STRANGE 'CLICKING' SOUND. DETERMINED IT WAS NOT AIRFRAME OR ENG; INVESTIGATED RADIOS. TURNED EACH OFF MOMENTARILY; NO LUCK. FINALLY DETERMINED PROB CAUSED BY A HEADSET MIKE A PAX HAD REMOVED AND HUNG UP WHILE NAPPING. VIBRATION CAUSED MIKE TO OCCASIONALLY STRIKE THE HARD SHELL OF THE HEADSET PHONES THEREBY MAKING A SOUND AND BREAKING SQUELCH. I THEN NOTICED I HAD NEGLECTED TO SWITCH BACK ON THE RADIO I WAS WORKING CTR. I CALLED CTR; CTR SAID THEY HAD BEEN TRYING TO REACH ME; THAT I WAS 'WELL BELOW' MEA: CLB TO 13000 FT; BECAUSE OF EXCELLENT WX I HAD THE TERRAIN IN SIGHT AND WAS SAFELY ABOVE IT. THOUGHTS: I WAS PROBABLY 'OFF THE AIR' SEVERAL MINS DUE TO MOMENTARILY FORGETTING TO SWITCH THE RADIO BACK ON: NOT HARD TO DO WHEN PREOCCUPIED WITH AN UNUSUAL PROB. WHILE THE PROB AND ITS SOLUTION DID NOT SEEM THREATENING; I PROBABLY SHOULD HAVE INFORMED CTR WHAT I WAS DOING. BECAUSE OF GOOD WX; THERE WAS NO DANGER; BUT CTR COULD NOT KNOW THIS.