Drone-rockers Sigur Ros are introducing pop fans to the power of singing sans words. But composer-dancer Meredith Monk has been plumbing huge emotions from exquisite babble for over 30 years. The score to a multimedia piece, mercy is quintessential Monk: transcendent chamber music that morphs between medieval polyphony, what sounds like Balinese monkey chants, and the minimalism of New York City peers like Steve Reich (whose early work with voices and mallet instruments is an influence here). The piece deals with a topic sadly out of vogue lately. But one can always hope for a comeback. WHEN WORDS FAIL At a Sept. 11, 2001, performance in Germany, Bjork sang Monk’s wordless signature ”Gotham Lullaby” in response to the day’s tragedy.