The Milk Carton Kids’ “Prologue” was released in Los Angeles in the middle of July last year, but you’d never guess it from the mournful, wintry vocals and Appalachia-tinged folk guitar that characterizes the album. (Even the album art is a snowy urbanscape.) The boyish, mild-voiced duo has been on a meteoric rise for the past year, distinguishing themselves among a field cluttered with folk wannabes. Their success is undoubtedly due to the flawless guitar playing of Kenneth Pattengale and Joey Ryan, who achieve the clean, weeping sound of traditional American folk that can resonate — unironically — with even the stoniest hipster heart. Bright, featherlight playing lightens the gloom in songs like “Undress the World,” but most of the album is devoted to the kinds of songs that evoke the exquisite depression of old breakups and rainy days. On first listen, the almost sibling-like singing sounds a little milquetoast compares to the robust sobbing of the guitar, but hang in there. You’ll catch the plaintive tones of Simon and Garfunkel at times, the twang of the Jayhawks at others. And the lyrics belie themes more mature than you might expect at first from the tender-voiced duo. “I don’t feel the pain I once did / One day just finished like a milk carton kid,” they intone on the eponymous track “Milk Carton Kid.” “All your rooftops set free in a hurricane wind / I don’t feel the pain I once did.” It’s a richly emotional album, a beautiful soundtrack for those forays into self-pity. Definitely worth the download, which you can find for free on their website (www.themilkcartonkids.com).