The tone of the initial line of dialogue until "Gettysburg" is reassuring, and it utilizes punctuation as a clue. The tone best suited for reading the word "bang" is excited.
The sequence of events in Twelfth Night from the earliest to the latest:
1) Olivia turns down Orsino, expressing a preference for Cesario.
2) Antonio takes a risk by bringing Sebastian to Illyria.
3) Olivia confuses Sebastian for Cesario.
4) Viola is shocked to see her brother alive.