FLC OF AN LTT HAS AN ERRATIC VOLUME CTL PROB ON COM RADIO. WHILE ENGAGED IN CONVERSATION WITH FO; THE CREW FAILS TO HEAR ATC CALLS. THEY FINALLY REALIZE SOMETHING IS WRONG AND CONTACT ATC.

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CRUISE FLT AT FL180 FROM VLD TO ATL. I HAD NOTED ON AN EARLIER LEG THAT THE VOLUME CTL ON MY AUDIO PANEL WAS ERRATIC. THE VOLUME WOULD UNPREDICTABLY RISE OR FALL WITH NO INPUT FROM ME. MY FO AND I WERE ENGAGED IN AN INTENSE CONVERSATION AND I SUSPECT THAT MY VOLUME BECAME LOW AND THEREFORE I MISSED CTR CALLING US. AS THE ACFT NEARED MCN VORTAC (WHERE WE NORMALLY WOULD CROSS AT 11000 FT) I REALIZED THAT SOMETHING WAS WRONG. I STOPPED THE CONVERSATION AND CALLED ATC. THEY CONFIRMED THAT WE HAD MISSED SEVERAL CALLS AND PROCEEDED TO VECTOR US AROUND AS NECESSARY IN ORDER TO DSND US TO THE PROPER ALT FOR THE ARR. I APOLOGIZED AND TOLD THEM I HAD A SUSPECT AUDIO PANEL. THE REST OF THE FLT WAS NORMAL AND MY VOLUME CONTINUED TO FLUCTUATE. UPON LNDG; WE RPTED IT TO MAINT AND IT WAS EVENTUALLY REPLACED. THE MALFUNCTIONING AUDIO PANEL CONTRIBUTED TO THIS COM BREAKDOWN; BUT I AM ACUTELY AWARE THAT IF MY FO AND I HAD NOT BEEN SO INVOLVED IN OUR CONVERSATION THAT ONE OF US WOULD'VE HEARD ATC CALLING; OR I WOULD'VE NOTICED THE REDUCED VOLUME. ALSO CONTRIBUTING WAS MY DECISION NOT TO HAVE THE PANEL REPAIRED AFTER OUR FIRST BECOMING AWARE OF THE PROB. (I WANTED TO WAIT UNTIL WE HAD A BREAK LONG ENOUGH SO WE WOULD NOT BE DELAYED.) I THOUGHT IT WAS NOT SERIOUS AND THAT 'I COULD LIVE WITH IT FOR A ROUND TRIP OR TWO.' THIS WAS A VALUABLE LESSON AND I'LL ENDEAVOR TO AVOID SUCH MISTAKES IN THE FUTURE.