Jennifer Lopez has already moved beyond the confines of the everyday Hollywood sex symbol: She has become a symbol of sex itself, muscling her way into Forties-style silver-screen glamour and doing more for womanly hips than any movie star since Rita Hayworth got her Gilda on. So it makes sense for La Lopez to try her luck at music, too, as the old-fashioned kind of all-purpose entertainer for whom telethons and Bob Hope specials were invented. The happy surprise of On the 6 is that she knows what she’s doing. Instead of strained vocal pyrotechnics, Lopez sticks to the understated R&B murmur of a round-the-way superstar who doesn’t need to belt because she knows you’re already paying attention. When she gets the right support from a top-shelf song daddy like Rodney Jerkins, she’s all brassy, breathy confidence, evoking her great precursor Ann-Margret’s 1964 classic with Al Hirt, Beauty and the Beard. Behold jiggy Jennifer Lopez, song-and-dance woman: She makes a little va-va and a whole lot of voom go a long way.