Here's an essay by Rabbi Howard Jachter on The Parameters of Kol Isha. It's an interesting presentation of many sources.
I found it via this post on Hirhurim who is currently discussing the issue. I hope that "Simcha" covers this issue as thoroughly as he did the issue of Women's Prayer Groups, and I recommend taking a look at his archives for the posts. They are quite comprehensive.
A question for the rabbi. Would a synthesized female voice be assur as Kol Isha? Would there be a difference depending on how the synth generates sounds, for instance, via samples, FM, or additive synthesis?