No one could top the dance-floor peak of 1998’s ”Believe.” But at least Cher and her producers have found a respectable way to make sure the beat goes on with Living Proof, her follow-up. Hewing to formula, the album once again turns back time to the disco era, and ratchets up the vocal tricks of ”Believe” to the 10th power. True, Cher’s voice seems even more worked on than her face. But she still has feeling. And her songs have enough oomph to make this more than a one-shot comeback.