The MonkMakes Sensor Board for micro:bit allows you to sense sound level, temperature and light level.
The Sensor for micro:bit uses a MEMs (microphone on a chip) and a pre-amplifier. The output of the sound sensor is connected to an analog input where it can be sampled. The sound signal varies about the 1.5V level. So, silence will produce an analog output of around 1.5V. When there is sound the analog readings will oscillate above and below the 1.5V level like this: