The word "bang" appears nineteen times in the text, and not in any sort of context like a sentence, either. Instead, they're little interjections, words that erupt into the text when something important happens. So let's look at exactly where they show up:
The first "bang" appears when Macabéa's boss, Senhor Raimundo Silveira, warns her that she will probably soon be fired because he is tired of her typing mistakes. So, what we're getting here is that "bang" occurs after some disturbing (and probably surprising) news.
The second "bang" appears when the narrator announces to us that he is about to tell us Macabéa's history (3.60). This is a little different, but it still signals a shift of some sort. It tells us that something has changed.
And how check out the section in which they appear the most. Nine of the nineteen "bangs" appear in when Macabéa visits Madame Carlota, the fortuneteller. Here's a list of those "bang" moments in Chapter 5:
So, whenever "big" news is about to be announced or when Macabéa has a strong and new emotion or reaction, we get a "bang." They signal a change and a shift; we get the sense that Macabéa is one person before a "bang" and a different person after the "bang."
Oh yeah, and the final "bang"? Right before the yellow Mercedes runs her over.