The sonic aspects of Plato's analogy of the cave is taken as a starting point for thought experiments to investigate the objective nature of sound, and the idea of quasi-Platonic forms in music. Sounds are found to be objects in a way that sights or appearances are not, and it is only in the presence of technology that they become artificial. When recognition, control and communication about sound come into play, abstract concepts emerge, but there is no reason to give these the priority status Plato affords to forms. Similar issues arise in discussion of the ontology of musical works, where the ideas of extension and intension prove useful for clarity about the nature of musical objects. They are also useful for strategies in the development of music software. Musical concepts are not fixed but arise from complex cultural interactions with sound. Music software should aim to use abstract concepts with are useful rather than correct.