When someone is asking for a "Sequence of Operation", they are using fancy language for "What Happens?" The fire department, because they are concerned with fire safety, is asking you to say "When the fire alarm system goes into alarm, the sound system is turned off. And the sound system stays off until the fire alarm system is restored from a fire alarm condition".
In order for you to say that, though, you have to test this sequence. You have to set the fire alarm system into alarm, and confirm that the sound system actually turns off, and stays off.
Keep in mind, the music is going to be loud and there are flashing lights with the music. The fire department wants assurances, in writing, that the music is not going to prevent everyone from knowing that there is a real fire alarm.