Do you mean "beautiful" as in "I find this woman physically attractive," or "beautiful" as in "I think she has a beautiful voice"? I find all kinds of women's voices beautiful, from the rough edged, bluesy, smoky, soulful and sultry to the ethereal, otherworldly, high and girlish.
Some women in music who I think are attractive...
Beth Ditto
Liz Fraser
Kristin Hersh
Sleater-Kinney (sadly now defunct)
Jolie Holland
The Be Good Tanyas
...Seriously, how cute are they?
Oh, and how can I forget... Letitia Sadier from Stereolab? Gorgeous French woman.