I have listened to Emiliana, Sheila, and Isabel sing (not on the soundtrack, though). All three of them have beautiful voices.
I can't really describe Emiliana's voice. It's different than what you usually hear. But I think it will work well for Gollum's Song. Listen here:
Emiliana Torrini
I like Sheila's style. It's a mixture of Indian and Celtic and I think she will be very interesting to hear on the soundtrack. Listen here:
Sheila Chandra
What can I say? Isabel has a gorgeous voice. It sets the tone of Middle-earth. I can totally imagine her singing Elvish. Listen here:
Isabel