Primitive play-once waits until pending events on all audio components have finished executing before itself executing the event specified by the current point in sound-space. The action executed by play-once could itself be either synchronous or asynchronous. In either case, the duration of the event is specified explicitly as a time unit or implicitly by the nature of the event. Primitive synchronize-and-play is similar, except that the sound to be played is specified explicitly.
AsTeR typically uses this primitive to generate sounds to cue the beginning of the rendering of certain objects.