The above is a clip from the documentary (of sorts) Baraka. A beautiful film, but this is the part that sticks with most people. It shows a Kecak, a form of dance created in the 1930s in Indonesia. It comes from a more traditional form of trance-inspired dance.
Whenever I think of this (or of other places where people congregate to sing in unison) I inevitably turn to Durkheim's notion of collective effervescence, a term for a shared emotionality that lends itself toward belief in the sacred. https://en.wikipedia.org/wiki/Collective_effervescence