This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of pleasant? It is also pleasant.
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.