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Yes, first and second crack are audible. It sounds like popcorn. First crack is caused by a build up of water vapor inside the cells of the coffee seed which eventually gets to a high enough pressure to rupture the cell walls. Second crack is the same thing except there it's mostly carbon dioxide gas. The more uniform the green coffee is, the shorter the duration of either event (leading to a greater lull between the events). If you're pushing a particularly aggressive roast with a non-uniform coffee the two can end up running into each other, but they'll sound a little different from each other. Second crack relies on chemical changes that are happening after the start of first crack while first crack is largely driven by physical changes that happen earlier so a well placed temperature probe also makes it easy to tell which of these is happening. If you stick to good coffees those measurements are pretty consistent.