Please...I want to know this :)

In Liberian Girl sang a south african singer named Letta Mbulu.
In Heal the world at the end is the voice of Christa Larson.
In History I don't know.. sorry :(

About Naomi.. I don't think they were more than friends.

well I really don't know about the singers...thank You liberiangirl_mj for telling us!

But as far as in the closet is concerned, i think they only had a professional relationship. They worked together for the video, and were friends and when the video was over would have said good-bye to each other. But i think, Michael was dating Lisa at the time..

So no Naomi and Michael didn't have any relationship.

in Liberian Girl it was Letta Mbulu
HIStory & Heal The World ; I don't know .

& about Michael's and Noami relationship . . they never had anything together . Because Michael started to hate her after she said to him during the shoot of the video that she wanted to suck his penis .

Letta Mbulu is the one who sung "Naku Penda pia, Naku Taka pia, Mpenziwe!" (I love you too, I want you too, my love!)

Heal The World:
Christa Larson: Ending solo vocal
Ashley Farrell: Playground girl

HIStory is unknown sorry.

And what MJ & Naomi did behind the scenes isn't known. Naomi claimed they kissed and that's not hard to believe. They were all over each other in the video and in the making of. They were seen out again on several occasions.

I mean look at this, he's all over her here:
link

That was from Dinner with Michael Jackson I believe. Some contest where fans got to have dinner with him lol it was during the filming of In The Closet I believe also. It'll be cool too see all the footage from the Dinner with MJ.

Them at some award place:

link