In the latest issue of You magazine - a free supplement to Daily Mail and Daily Irish Mail - Annie Lennox mentions Madonna's involvment in "Sing":

To that end, "Song of Mass Destruction" (Annie's upcoming album) features a female-empowerment anthem called "Sing", which will be available in download and single formats, with all profits going to South Africas's Treatment Action Campaign (TAC) initiatives.

Its rousing chorus features 23 of the planet's top female singers - Celine Dion, Joss Stone, Madonna, Dido, Fergie and Shakira, among others - dubbed "the choir of 23" by Lennox.
She sent out a "little letter" requesting their services, "and everyone who was around came back and did it", she marvels.

"I asked them to join in the chorus, but Madonna sang the second verse as well."

Was that because she just couldn't bear to play second fiddle?
"No it was really cool of her," insists Lennox.
"Look, I'll be as mercenary with this as I need to be. Madonna has a huge audience, and her involvment will help to spread the word wider."

Source: MadonnaTribe