Audio-visual performers create holistic total artworks linking sound and image, in part to address the perceived lack of visual feedback in electronic music performances. We suggest that Audio-Visual User Interfaces (AVUIs), extending graphical user interfaces by adding interrelated sound and image, can be an important element to create these works, while also conveying additional feedback to audiences. We present AVUI and situate it among two other types of interfaces connecting sound and image in audio-visual performances: visualised live coding and augmented tangible user interfaces. We review each of these types of interfaces and identify the qualities required for a legible and expressive AV system. We then present the Enabling AVUIs project, which sought to facilitate the design and devolvement of audio-visual interfaces. The project deployed a user-centric design process, engaging with a community of performers. We present the project outcomes and propose guidelines and a toolkit for AVUI development. We assess strengths and weaknesses of AVUI compared to other interface approaches and discuss best practices in AVUI design for audience understanding. This analysis allows us to map the territory of interfaces for audio-visual performance and to adapt the user-centred design model for toolkit design.