The Swedish metal band Opeth widened its sound in the mid-’90s, letting in acoustic guitars and clean vocals, and its albums since then have been careful refinements, with long, admirably coherent songs. “Watershed” (Roadrunner), the new Opeth album, broad enough to encompass death-metal pummeling as well as cello and English horn, is typically engrossing — symphonic, and in a way organic. On songs like “The Lotus Eater,” the music keeps revealing and then covering up the acoustic-folk tissue that’s under the metal skin. Then there’s the flip side of organic music: decomposition. At the end of “Burden,” a minor-key, prog-metal ballad, the band fades out to leave two acoustic guitars, playing folkish patterns in counterpoint. Then one of those guitars drops out; the remaining guitarist keeps playing his finger-picked pattern as someone else slowly detunes his guitar, for a full minute.