A simile is used to compare her voice to a "cool, dark room in summer- peaceful, soothing, quiet". This helps let readers know that she is most likely very nurturing, caring, and a definite "motherly figure"
A simile is a figure of speech where two things are directly compared and one is an exemplification of the other, it is widely used by writers because it helps to give specific qualities and characteristics to the people and places in the story, here the simile is describing the voice as something peaceful soothing and quiet.
