ACR X NON ADHERENCE TO ATC CLRNC. UNAUTH DSCNT BELOW ASSIGNED ALT. ALTDEV.

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ON DSCNT INTO SUN I UNDERSTOOD ATC TO CLR US FOR A VISUAL APCH. I DSNDED THROUGH OUR PREVIOUSLY ASSIGNED ALT OF 13000 FT MSL FOR THE APCH. ATC ADVISED US OF TFC LEAVING SUN AND THEN ISSUED HOLDING INSTRUCTIONS AT HLE NDB. WE THEN CANCELED IFR AND PROCEEDED TO THE ARPT AND LANDED UNEVENTFULLY. IT IS NOT KNOWN IF WE WERE ACTUALLY CLRED FOR A VISUAL APCH OR IF WE MISUNDERSTOOD THE CLRNC AND DSNDED THROUGH OUR ASSIGNED ALT. POSSIBLE CAUSES: LACK OF CONCURRENCE OF A CLRNC BTWN MYSELF AND THE CAPT. POOR COM BECAUSE WE WERE USING PUSH-TO-TALK INTERPHONE SWITCHES. SUPPLEMENTAL INFO FROM ACN 294656: IN ACCORDANCE WITH THE ACR X DHC-8 CDL 1 NOSE LNDG GEAR DOOR HAD BEEN REMOVED. THIS PRODUCED A LOUD HOWL IN THE COCKPIT ABOVE 180 KIAS. AT APPROX 220 KTS THE NOISE GOT LOUDER AND ASSUMED A LOWER PITCH. ALSO; THIS WAS OUR FIRST LEG IN THE ACFT AND I WAS UNAWARE OF THE LIMITATIONS THIS WOULD PRODUCE (NOISE WISE) UNTIL WE WERE IN CRUISE FLT. WHEN WE DISCOVERED THAT THE NOISE WOULD PRESENT A PROB FOR INTERCOCKPIT COMS MY FO AND I DECIDED TO USE THE COCKPIT INTERPHONE SYS. NORMAL COCKPIT COMS ARE USUALLY VOICE- VOICE AS THE NOISE LEVEL IS SUITABLE FOR THIS. THE COCKPIT INTERPHONE SWITCH IS THE SAME SWITCH AS THE BOOM MICROPHONE SWITCH ON THE CTL YOKE. PUSH L AND IT ACTIVATES THE BOOM MICROPHONE; PUSH R AND IT ACTIVATES THE INTERPHONE. I WAS USING A DAVID CLARK H-30 AND THE FO WAS USING A 2 EARPIECE TELEX LIGHT WT SYS. EVERY TIME I NEEDED TO COMMUNICATE WITH THE FO; I WOULD HAVE TO PUSH THE INTERPHONE SWITCH. IT IS A MOMENTARY SWITCH AND DOES NOT LOCK IN PLACE. SOME OF OUR LATER MODEL DHC-8 HAVE A HOT MIKE SYS. EVEN THOUGH IT IS RARELY USED; HAD IT BEEN INSTALLED IN THIS ACFT; I DOUBT THE EVENT WOULD HAVE EVER HAPPENED. THE OTHER CONTRIBUTING FACTOR WAS THE OUT OF 'NORMAL' SEQUENCE OF THE CHKLISTS. PER OUR FLT STANDARDS MANUAL THE DSCNT CHKLIST IS DONE BELOW 18000 FT AND BEFORE 10000 FT. THE 10000 FT CHIME OCCURS AT 10000 FT AND THE APCH CHKLIST IS ACCOMPLISHED AT 5000 FT AGL OR 5 MINS FROM LNDG. THIS MAKES IT NECESSARY TO DO THE DSCNT AND APCH CHKLIST OUT OF SEQUENCE ABOVE 10000 FT. IT IS TYPICAL TO HAVE THE DSCNT CHKLIST; 10000 FT CHIME; APCH CHKLIST; AND APCH CHIME DONE ABOVE 15000 FT. HAD WE NOT HAD THE COMS PROBS IT PROBABLY WOULD HAVE BEEN A NORMAL MLS STYLE ARR. OPERATING AS ACR X WE HAD BEEN CRUISING AT 16000 FT (SEVERE CLR). THE FO (PF) ASKED FOR AN INITIAL DSCNT FOR THE ARR INTO SUN (HAILEY; ID). WE WERE ASSIGNED 13000 FT. I BEGAN THE DSCNT. AROUND 14000 FT I CALLED THE ARPT IN SIGHT FOR A VISUAL APCH. ATC REPLIED SOMETHING TO THE EFFECT; EXPECT HOLDING AT HAILEY (NDB); IFR TFC DEPARTING SUN VALLEY; RPT HIM IN SIGHT. SHORTLY AFTER THIS; THE FO CALLED FOR THE APCH CHKLIST. HE ALSO SAID; 'LET'S CHIME THE FLT ATTENDANT NOW FOR 10000 FT' (CHIME FOR STERILE COCKPIT). THIS OCCURRED AT APPROX 13500 FT. I BEGAN THE CHKLIST. THE FO APPARENTLY MISTOOK OUR LAST XMISSION WITH ATC AS A VISUAL APCH CLRNC. AT APPROX 12500 FT I NOTICED HE HAD FLOWN THROUGH OUR ASSIGNED ALT OF 13000 FT. I ATTEMPTED TO VOCALIZE THIS TO HIM (DID NOT USE INTERPHONE). DUE TO THE NOISE IN THE COCKPIT HE DID NOT UNDERSTAND WHAT I WAS SAYING. IN A LAST DITCH EFFORT I ATTEMPTED TO CALL ATC AND CANCEL IFR. I ENDED UP TRYING TO XMIT TO ATC ON THE INTERPHONE. I FINALLY FIGURED OUT MY MISTAKE AND WE CANCELED IFR AT 12000 FT. DUE TO INTERMITTENT RADAR COVERAGE IN THIS AREA ATC PROBABLY NEVER SAW THIS DEV.