Celeste Mendoza, who was born in Santiago de Cuba, sang ballads with a convincing heft in her voice, a depth of emotion that made them work. But it was her singing of guaguancos, the music of the street rumbas, that made her famous. When she sang them -- a good example is her version of ''La Confianza,'' which she recorded with Bebo Valdes's orchestra -- the sadness in her voice and her smooth virtuosity mixed well. And they earned her a place in the history of the music: previously