POP/ROCK. Roberta Flack - Roberta. (Atlantic ): Roberta Flack's latest album is way past due, and it announces a rich new approach to some old favorites. Flack's voice is a pitch lower, and it's lost the silky, overly smooth sound it had when she owned the charts for overly produced slow ballads. Now there's a smoky, more-intimate quality, and she's discovered Rogers and Hart, Ellington, and people even more proven by time. The selection, though varied, seems to work together for a wonderful album. ``Prelude to a Kiss,'' ``Tender- ly,'' and ``My Romance,'' are right next to ``Thrill is Gone'' and ``Sweet Georgia Brown.'' She even asks ``Isn't it Romantic?'' Well, yes, it is.