This word's reading is pretty weird. We're going to think back to a couple different words to put this one together. The word for heart (心), is こころ. The word for good is よい (or いい, but for this word's sake, よい). A "good heart (feeling)" is one where your heart feels nice and warm and fuzzy (figuratively, not literally. If it's literal, go to the hospital please). That is a pleasant feeling. A こころよい feeling.
Her voice is very pleasing to the ear, like the babbling of a brook.