The use of metaphor has a prominent role in HCI, both as a device to help users understand unfamiliar technologies, and as a tool to guide the design process. Creators of new computerbased instruments face similar design challenges as those in HCI. In the course of creating a new piece for Mobile Phone Orchestra we propose the metaphor of a sound as a ball and explore the interactions and sound mappings it suggests. These lead to the design of a gesture-controlled instrument that allows players to "bounce" sounds, "throw" them to other players, and compete in a game to "knock out" others’ sounds. We composed the piece SoundBounce based on these interactions, and note that audiences seem to find performances of the piece accessible and engaging, perhaps due to the visibility of the metaphor.