Sonic Structures is a multi-user mixed reality experiment that explores the relationship between visual form and sound and music creation through the lens of 4E (embodied, enactive, embedded and extended) cognition. By extending a custom platform allowing large scale mixed reality experiences for up to 20 users with real-time audio processing and generative 3D graphics, Sonic Structures provides a sandbox environment for the real-time transformation of sound and music into persistent visual structures. Described here is the conceptual and architectural design of the project and the results of a workshop exploring the musical, educational and artistic potential of the system.