Since accommodating all the singers would have posed a scheduling challenge, Lennox asked them to contribute vocals on the chorus of the song. When she received the recorded vocals from Madonna, Lennox found that Madonna had not only sung during the chorus, but also contributed by singing the second verse. She "was really touched – for Madonna is very rigorous in what she gets involved in and for her to do that for me, I was thrilled to bits". Lennox also admitted that featuring Madonna would bring a bigger audience for the song, thereby helping the cause. The refrain consists of an "empowering message" with the group of singers belting the line "Sing my sister Sing! / Let your voice be heard" while interpolating "Jikelele" in between. According to Jon Pareles of "The New York Times", "Sing" consists of a piano played in the background reminiscent of Marvin Gaye's 1968 single, "I Heard It Through the Grapevine". In an interview with "Performing Songwriter" magazine, producer Glen Ballard recalled that he completed the recording of the featured artists in various locations, conducting the sessions through the Integrated Services Digital Network (ISDN): "Shakira in Puerto Rico, Pink in Zurich, Madonna in London etc". Lennox contacted British Indian musician Nitin Sawhney to work on an alternate version of the song. Her official website also announced the release of special remixes of the track on 1 December 2007, with contributions from Moto Blanco, Dean Coleman and Harry "Choo Choo" Romero among others. Thom Jurek from AllMusic described "Sing" as a "huge feminist anthem" and described it as consisting of a "killer hook, a big bad soul/gospel refrain, and a beat that, once it gets into the spine, will not be easily dismissed". "Boston Globe"s Sarah Rodman compared it to the Eurythmics' 1985 single "Sisters Are Doin' It for Themselves" adding that it carried Lennox's characteristic messages of "optimism and empowerment". Liz Hoggard of "The Guardian" called the song a "showstopper" and the collaboration as "incandescent". Sal Cinquemani of "Slant Magazine" also noted the anthemic nature of the track and its "smartly plays more like a broad-spectrum sisters-are-doin’-it-for-themselves" nature, while believing that it would have been better suited as the album's title song than "Songs of Mass Destruction".