This thesis looks at the requirements analysis, design, development and evaluation of an application, CymaSense, as a means of improving the communicative behaviours of autistic participants through therapeutic music sessions, via the addition of a visual modality. Autism spectrum condition (ASC) is a lifelong neurodevelopmental disorder that can affect people in a number of ways, commonly through difficulties in communication. Interactive audio-visual feedback can be an effective way to enhance music therapy for people on the autism spectrum. A multi-sensory approach encourages musical engagement within clients, increasing levels of communication and social interaction beyond the sessions. Cymatics describes a resultant visualised geometry of vibration through a variety of mediums, typically through salt on a brass plate or via water. The research reported in this thesis focuses on how an interactive audio-visual application, based on Cymatics, might improve communication for people on the autism spectrum. A requirements analysis was conducted through interviews with four therapeutic music practitioners, aimed at identifying working practices with autistic clients. CymaSense was designed for autistic users in exploring effective audio-visual feedback, and to develop meaningful cross-modal mappings of musical practitioner-client communication. CymaSense mappings were tested by 17 high functioning autistic participants, and by 30 neurotypical participants. The application was then trialled as a multimodal intervention for eight participants with autism, over a 12-week series of therapeutic music sessions. The study captured the experiences of the users and identified behavioural changes as a result, including information on how CymaSense could be developed further. This dissertation contributes evidence that multimodal applications can be used within therapeutic music sessions as a tool to increase communicative behaviours for autistic participants.