"Joan Baez's Greatest Hits." Vanguard. There was a time, before the arrival of Emmylou Harris and Maria McKee, when I thought Baez had the loveliest voice in all of pop. And there are moments on this 16-song collection of works from the late '60s and early '70s when she still sounds like an angel. But these tracks--ranging from "Help Me Make It Through the Night" to "Put Your Hand in the Hand"--haven't held up well. Her interpretations are sometimes spiritless and a few of the pop-oriented arrangements are downright hokey. Why doesn't Vanguard release her early '60s LPs?