Plena Libre is a 10-year-old, 12-member band nominated for three Grammy awards for its refreshing updates of plena, bomba and other popular sung dances.  “Estamos Gozando!” is the ensemble’s spiffy, sterling tribute to revolutionary composers of Afro-Rican standards.  Standouts include a very hip version of “Que Buena Son Las Mujeres” by trumpeter Cesar Concepcion, a big big-band arranger in the 1960s, and a very jazzy version of “Lluvia con Nieve” by Mon Rivera, who introduced four trombones to folk orchestras.  Manuel Jimenez (“El Canario”), the first commercially recorded plenero, would have enjoyed the agile, sexy take on his “Lo Que a Ti Te Gusta,” which features a small accordion called the sinfonia.  Group leader Gary Nunez’s arrangements are sleek without being slick.  The percussionists are splendidly shifty, the call-and-response vocalists are unusually conversational and the horn players and pianist Luisito Marin carry on a rich courtship.  All in all, “Estamos Gozando!” is free-form fun.