Honestly, I think that you are doing all the right things by talking to her about the alarm. I think that eventually, your daughter will come to cope with her anxiety regarding the alarm. Only the decision his hers as to when to cope with it, not yours.
I think that it is relatively normal, especially for kids, to become obsessed or consumed with anxiety when presented with a scary or unusual event or experience from time to time. If you feel that her anxiety is much too excessive or has been lasting for much too long, you might want to consult with her pediatrician to determine the best course of action which could be in the form of anti-anxiety medication and/or talk therapy. Other than that, you might need to wait it out until your daughter is ready to cope with her anxiety in relation to the alarm.
Thanks...She's already done that and know the importance of the alarm, she just can't let go of the sound.
Take her to the fire station and let the firefighters there give her a tour of the equipment.