Edmar Casteñeda, a young harpist from Columbia, has yet to record his first album. But the good word on his sextet (with drums, congas, bass, flute and trombone) is flying around town so fast that the 25-year-old’s April residency at Sweet Rhythm (88 Seventh Ave. South, off Bleecker Street; [212] 255-3626) has been extended through May. The artist, who treats his harp like a piano or guitar, performs Sunday as well as May 8, 27 and 28. Tickets are $15 and there’s a $10 drink minimum.