The LM393 dual comparator-based sound sensor provides two outputs, one analog (typically A0) and one digital (typically D0). While the sensor's digital output (D0) is connected directly to the OUTPUT pin of the LM393's first operational amplifier, the sensor's analog output (A0) is connected to the microphone via a resistor (150R-1500k) BEFORE the comparator so its value is not determined or modified at all by the LM393.
I suspect that the issue mentioned by Kostantinos has to do with the way Arduino converts the analog signal coming from the microphone into a numerical value (typically between 0 and 1023). The value read at the sensor's analog output (A0) when the microphone does not register any sound ("silence"), will change (usually decreasing) over time depending on the variations of sound level. I guess the mentioned issue appears when trying to use a fixed value (say 500) as a threshold to determine whether there is sound or not because the actual threshold must be adjusted over time.