The S5D9 IoT Fast Prototyping Kit ships with a clear sticker covering the microphone to protect the microphone from dust. Removing the protective sticker will improve microphone sensitivity.
Just found this out at the Learn IoT community NY meetup this past week. When you unpack your S5D9 board, take a close look at the sound sensor. If you see two red bars, that means there’s a small plastic cover on it. It will still detect sound, but if you take it off, it becomes more sensitive. Just use your fingernail, it comes off relatively easily.