While each person is playing their part, others need to listen quietly to what they are playing and let them concentrate on the process. As in the video, have each student create 3-4 different choices for the 'producers' to decide from. Try to create a feeling of building consensus rather than creating competition when voting on parts which should be used for the composition. A competitive situation will quickly become a forum for the class leaders to completely influence the rest of the class' decisions. Explain that the song will turn out the best if everyone votes for what they really feel is the best part, rather than what their friends think is the best part.
All answers are right and everyone can stay friends.
As the class picks each choice for the song, refer often to the type of song the kids decided to write. Often, parts will be played or decisions made which can steer the piece away from the intended goal. By referring to the style the students initially chose, you can guide their choices and stay on track. Sometimes it's fun not not stay on track and let the creative flow guide the piece. Then, compare the end product to what had been intended.