Yes (not all of them)
Sometimes it's the actual timbre of the voice that's attractive, sometimes it's the way they use it.
With Suzanne Vega, it's the timbre, at least in some of her songs, like Tom's Diner. Her voice in that song reminds me of "home" in some spiritual sense.
With Mila Kunis, it's the way she uses her voice. I've had a huge crush on her ever since That '70s Show for that reason. (Nowadays she plays Meg on Family Guy.)
With Sarah Silverman, it used to be the way she used her voice. Once I heard a cartoon with a guest character in it, and I was so enamored with the way that character talked that I looked up who played that character on that episode of that obscure TV show (can't believe I actually found it), and it turned out it was Sarah Silverman, which was both surprising and not--I'd already known I loved the way she talked at that time.
With Nadia Ali, it's the way she sings. OMG, the timing, the intonation, the voice cracks, it's so perfect. iiO - At the End
With Justine Suissa, it's maybe a combination of both her actual voice and the way she sings. There's a reason people love to hear her in chillout music, though maybe it's a bit harder to put a finger on than in Nadia Ali's case. Above & Beyond pres. OceanLab - Miracle, Out There (5th Dimension) (Christian Rusch's Chillout Mix)
And here's the most adorable voice IN THE WORLD EVER: Pop On Rocks (A Dr. Seuss Rap) - Holo Bass feat. Amelia Watson, Holo Bass Feat. Amelia Watson - Green Eggs & Spam