The other day, Mulan and I were having a chat about female singers with distinctive voices. Whitney Houston's name came up; so did Sade Adu's, as did Tracy Chapman's. The discussion got me thinking about other female singers with voices which were so distinctive that you could spot them in a hundred-person choir. It's not as easy as it sounds, because most people trying to make a career of singing tend to model their voices on successful singers' voices rather than doing their own thing. But two...