The first one I've measured is my wife's favorite singer, Happy Rhodes. I'm using a program called "Transcribe" from a company called Seventh String Software. It's usually intended for people trying to transcribe guitar solos. It displays a waveform envelope; the X axis is frequency, and the Y axis is amplitude; and a piano keyboard right underneath it. So if someone plays or sings middle C, there will be a big peak in the graph right above that key. If you check the Talk page on Happy's Wikipedia entry, you'll see a discussion involving myself (k8fan), my wife (xenussister) and Ben Wing.

http://en.wikipedia.org/wiki/Talk:Happy_Rhodes

I wish there was an open source program to accomplish this, but I've not found one.

That is an amazing project! Do you have a page for your work on wiki? I'd be interested in knowing how you are going about it, and what some of your findings are!

In reply to your comment:
“I'm starting a project to actually measure the vocal ranges of various singers for their Wikipedia entries. It's amazing how many 4 octave ranges become 3 and change when actually measured - like those seven foot tall guys in the NBA. But Yma does have an amazing voice. ”