PMDG 737-600 Fire Alarm Bell

Hello,I'm still enjoying flying the 737-600 but it seems upon battery, standy power switch and ground power connection I am missing the fire alarm warning bell when testing the fire warning system.The fire handles illuminate but I receive no aural sound, on a rare occasion I will hear the alarm but still have no reason why I hear it at certain times and other times I dont. I have been using the same start up list for many months and have tried different scenarios found by doing a search. I am currently using a two monitor setup with my overhead undocked, can anyone help?Many ThanksJustin Harvey