Slightly adapted from "Busca tu cancion" in 101 juegos musicales.
Choose three or four songs that are probably known to everyone in the group and write each song's title on two index cards. (Or ask each participant to choose a song that they think everyone will know, writing the title on two index cards.) You should have as many cards as participants. (For odd numbered groups, add an extra card for one song.)
Shuffle the cards and have participants each choose one card, being careful not to show it to others.
When the Joker says, "go!" the participants wander the room singing (or humming, or playing on an instrument) their songs until they find another person singing the same song. When they find each other, the pairs continue singing until all participants have found a partner.
When done, the joker can ask one pair to sing their song while another pair improvises a dance to it, and a third group improvises music (making sounds like drums, violins, guitar, trumpet, etc). You can switch roles, and repeat until all songs have been performed.
The activity can be used to form pairs for a new activity (interview, etc).
In language classes, or in situations where participants don't know all the words, they can sing as much as they know and just hum the rest.