ACR LOSES COM WITH ATC WHEN OPERATING WITHOUT HEADSET AND SPEAKERS ARE TURNED DOWN.

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NW OF BTY VOR (BEATTY; NV) GIVEN CLRNC BY ATC TO CROSS FUZZY INTXN AT 16000 FT AND 280 KTS. FO READ BACK INSTRUCTIONS. HE THEN WENT OFF #1 RADIO; USING #2 RADIO TO GET LAS ATIS AND COMMUNICATE WITH COMPANY. MEANWHILE; I WAS MONITORING #1 RADIO USING THE SPEAKER (FO HAD SWITCHED TO HEADSET FOR COM). AFTER SETTING 16000 FT IN ALT ALERTER AND CHKING THE FMC FOR XING RESTR AT FUZZY; I NOTED THAT THE RESTR HAD DROPPED OUT. BECAUSE 16000 FT WAS IN THE ALT WINDOW; THE NEW CRUISE ALT IN FMC WAS 16000 FT AND THE FMC WOULD NOT ACCEPT A 16000 FT XING RESTR. IT TOOK ME A SHORT PERIOD OF TIME TO FIGURE OUT WHY THE COMPUTER WAS NOT ACCEPTING THE RESTR. AT THIS POINT WE WERE APCHING FL280 SO I LEVELED AT FL280; INPUT FL280 IN FMC AS CRUISE ALT; REINSERTED 16000 FT XING RESTR WHICH WORKED. I THEN RESET 16000 FT IN ALT WINDOW. AT THIS TIME I PUT MY HEADSET ON TO HEAR CTR CTLR CALLING OUR FLT. I RESPONDED; HE ISSUED INSTRUCTION TO DSND TO FL270 AND CROSS FUZZY AT 16000 FT. HE THEN SAID HE HAD MADE SEVERAL CALLS TO OUR FLT. AT THIS POINT I REALIZED THAT THE VOLUME ON MY SPEAKER WAS TURNED DOWN. (I HAD BEEN HEARING THE FO'S DURING THE FLT PRIOR TO HIS SWITCHING TO HEADSET.) I WOULD HAVE CAUGHT THIS SIT SOONER HAD I NOT BEEN REPROGRAMMING THE COMPUTER. IF THE STAR CHART HAD REFLECTED THE XING RESTR I WOULD HAVE PROGRAMMED IT IN EARLIER IN THE FLT AND AVOIDED THE CONFUSION. ALSO; FO WAS HAVING DIFFICULTY PICKING UP LAS ATIS DUE TO TERRAIN AND THIS SIGNIFICANTLY INCREASED THE TIME HE WAS OFF CTR FREQ. THE A320'S RADIOS DO NOT HAVE SQUELCH CAPABILITY; WHICH I THINK WOULD AID IN THIS SIT.