Listening a few days ago to some early tracks by Joan Baez (on the Woodstock DVD) I realised just how beautiful her voice was then. I don't know of any other singer before or since with quite the same quality. I don't just mean it was a good voice. There are many good voices, - Ella Fitzgerald, Sarah Vaughan, Eva Cassidy, Kate Bush, Aretha Franklin - each with a distinctive timbre that identifies it within a couple of notes. In Joan's voice however there are no overtones - it gains its identity from its purity and clarity.
At Woodstock for example she sang the Ballad of Joe Hill giving the song an intensity that transcends mere performance. Two other favourites of mine are traditional songs (from the Joan Baez Ballad Book of 1972 - on vinyl) 'Go away from my window' and 'Barbara Allen' again both sung with an intense directness and purity.
At the other end of the spectrum is Billie Holiday. One of the most beautiful and affecting albums of all time is Lady in Satin. Here the 'voice' has almost gone, all that is left are overtones and resonances of a pretty desperate life. Listen to 'You've Changed' or 'Violets for my furs' to see what I mean.
I don't know why two such different voices can both be called beautiful. Perhaps in Billie's case it is the ability to hear what she was, still buried in the voice we hear now as the beautiful woman is buried in Rodin's sculpture of the old woman "She Who Was the Helmet-Maker's Beautiful Wife"