Why were the crew members speaking in hushed tones in the crew's quarters?-Charlotte doyle

You can find the answer to this question in Chapter 9, with some insight from Chapter 8. In Chapter 8, Charlotte writes that the captain had asked her to report back what she heard the crew saying to each other. Dutifully, Charlotte tells the captain that the crew is tired and disgruntled, and the captain promises her that they will soon have rest. In Chapter 9, however, Charlotte hears deliberately "muted" conversation from within the forecastle and strains to make it out.
What she hears is alarming: a number of crew members, at least four, are discussing having put down their mark in favor of something. Charlotte also hears them remark that they dislike her hanging around them and seemingly "spying," reporting what she hears back to their captain. Eventually, after having thought about it for some time, Charlotte realizes that she has observed the crew men plotting a mutiny—something she then reports to the captain.