One of the more enriching elements currently broadening the borders of pop is the desire of classically trained musicians to utilize their rigor on more whimsically intrepid projects.

Case in point is the all-women string quartet Osso, whose members have worked with artists ranging from Jay-Z to Antony and the Johnsons to Burt Bacharach to the New Pornographers. Now by itself, the only thing proven by that eclectic list is that almost everyone likes a little string sweetening — and that Osso can accommodate and help foster most any idiosyncrasy. The real splendor happens when the four cohere as the main event around a single, ambitious piece of music. And that’s what will happen tonight at the Southern Theater, when Osso performs songs from their just-released CD, “Run Rabbit Run,” a string-quartet rendition of Sufjan Stevens’ electronica project based on signs of the Chinese Zodiac.

What could be more chic and esoteric, eh? Except that “Run Rabbit Run” frequently prances and dovetails like the soundtrack to an animated cartoon, an avant garde for the kindergarten. It’s original and lacking in self-consciousness; not that surprising coming from the work of Stevens, who excels at making grand experiments feel casual. Check out Osso’s live version of “Year of the Boar,” from the group’s MySpace page, which also has a version of Stevens’ “Christmas In July.” And here is a video clip of Osso (with two different members), playing Stevens’ “Year of the Ox.”

As an added bonus, a showing of “The BQE” a 40-minute movie about the Bronx-Queens Expressway, filmed and scored by Stevens, will precede the Osso show. Stevens will introduce the movie, inevitably prompting speculation that he might later join Osso onstage.

An equally intriguing gig that is, alas, already sold out, is a performance by the Icelandic “folktronica” collective mum (pronounced “moom”) at the Walker tonight. As delicate and pretty as an 18^th century music box, mum’s tunes are derived from what the New York Times describes as “a melodica, a harmonium, and two PowerBooks.”

Iceland is renowned as a place for elves, ghosts, and fairies, and it’s not hard to imagine them championing the magical ethereality that is at the ever-dissolving heart of mum’s music. If you’re having a particularly stressful day and could use a grace note of sonic and visual beauty in your life, click on this video of mum’s “Green Grass of Tunnel.” Here they are live in Washington, D.C., performing “The Ghosts You Draw On My Back.”

Osso and Sufjan Stevens’ “The BQE” at the Southern Theater, tonight at 7:30 p.m., tickets $22. (Preshow reception at 6:30)

mum, with Sin Fang Bous at the Walker Art Center’s McGuire Theater,  tonight at 8 p.m., Sold Out. Waiting list tickets begin at 7 p.m. Tickets $18, $15 for members of the Walker or the Cedar Cultural Center.