BAE800 CAPT; DURING A PIC PART 135 CHK RIDE; OVERSHOT MISSED APCH ALT DUE TO DISTR ON GEAR WARNING HORN DUE TO SPOILERS SLIGHTLY OPEN; OR UP; AND THE MISPROGRAMMING OF THE PRESELECT AUTOPLT ALT.

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WE WERE GETTING A PART 135 CHK RIDE AND HAD REQUESTED MULTIPLE OPTION APCHS. OUR FIRST APCH WAS AN ILS TO RWY 30R WITH AN AMENDED CLB TO 4000 FT IN THE EVENT OF A MISSED APCH. LEVELING OFF AT THE 2500 FT GS INTERCEPT ALT; WE BEGAN CONFIGURING FOR THE APCH. SHORTLY AFTER THE GEAR WERE LOWERED; THE GEAR WARNING HORN SOUNDED (NON CANCELABLE); YET THE GEAR INDICATORS SHOWED 3 SAFE GEAR DOWN AND LOCKED. A MISSED APCH WAS EXECUTED BECAUSE OF THE UNKNOWN WARNING REASON. COCKPIT COM WAS DIFFICULT WITH THE HORN STILL GOING EVEN WITH THE GEAR UP. AT SOME POINT IN THE MISSED APCH IT WAS NOTICED THAT THE SPD BRAKE WAS 'CRACKED' OPEN (OFF THE MICRO SWITCH) SOUNDING THE HORN AS SOON AS FLAPS WERE EXTENDED PAST 15 DEGS. BECAUSE OF THE WARNING HORN AND ALL THAT WAS GOING ON; THE ALT ALERTED HAD BEEN SET AT 4700 FT FROM 2500 FT. BECAUSE OF THIS; THE ACFT CLBED THROUGH 4000 FT. THE PIC WAS QUICK TO NOTICE THAT THE ACFT WAS NOT CAPTURING THE ASSIGNED 4000 FT AND PUSHED THE PLANE OVER TO GET BACK TO 4000 FT. I THINK A NON CANCELABLE WARNING HORN HAS ITS PLACE (TO PREVENT GEAR UP LNDGS); BUT TO USE THE SAME HORN FOR SOMETHING THAT IS LESS NOTICEABLE (SPD BRAKE HANDLE JUST OFF THE MICROSWITCH AND A NON-LIT INDICATOR THAT SHOWS SPD BRAKE OPEN (VERY UNNOTICEABLE)) HAD THERE BEEN SOME OTHER EMER GOING ON; THE HORN WOULD CERTAINLY HAVE HINDERED IN ANY SOLUTION.