** Indigo Girls, “Shaming of the Sun,” Epic.

“Shame on You,” the hummable first single from Indigo Girls’ sixth album, perfectly showcases the veteran folk-pop duo’s charms. The staccato rhythm and impeccable harmonizing by Amy Ray and Emily Saliers give the tune a certain “Brown-Eyed Girl”-style charm. It’s too bad these qualities don’t prevail throughout “Shaming of the Sun.”

Co-produced by the duo with David Leonard, the album echoes the eclectic spirit of 1994’s “Swamp Ophelia,” offering selections as diverse as a Beach Boys-esque piano ballad and a hip-hop-spiked anthem. But aside from the single and the percussive “Get Out the Map,” the songs are drowned in a morass of instrumental bombast and overblown sentimentality.

The lyrics range from lost-love laments to social commentary, but the music’s stylistic puddle-jumping feels unfocused, despite expert support from such guest stars as Steve Earle, Ani DiFranco and violinist Lisa Germano. The Atlanta-based pair’s individual songwriting quirks run disastrously amok as well: Ray’s attachment to overwrought ‘70s guitar-god riffing yields the ponderous “Don’t Give That Girl a Gun,” while Saliers’ maudlin poetic tendencies sprawl to epic proportions in the grandiose, strings-infused “Caramia.” Memo to Indigo Girls’ next co-producer: When they get the urge to experiment, make them stop.

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