The combined use of sound and image has a rich history, from audiovisual artworks to research exploring the potential of data visualization and sonification. However, we lack standard tools or guidelines for audiovisual (AV) interaction design, particularly for live performance. We propose the AVUI (AudioVisual User Interface), where sound and image are used together in a cohesive way in the interface; and an enabling technology, the ofxAVUI toolkit. AVUI guidelines and ofx-AVUI were developed in a three-stage process, together with AV producers: 1) participatory design activities; 2) prototype development; 3) encapsulation of prototype as a plug-in, evaluation, and roll out. Best practices identified include: reconfigurable interfaces and mappings; object-oriented packaging of AV and UI; diverse sound visualization; flexible media manipulation and management. The toolkit and a mobile app developed using it have been released as open-source. Guidelines and toolkit demonstrate the potential of AVUI and offer designers a convenient framework for AV interaction design.