MoodMixer is an interactive installation in which participants collaboratively navigate a two-dimensional music spaceby manipulating their cognitive state and conveying thisstate via wearable Electroencephalography (EEG) technology. The participants can choose to actively manipulateor passively convey their cognitive state depending on theirdesired approach and experience level. A four-channel electronic music mixture continuously conveys the participants'expressed cognitive states while a colored visualization oftheir locations on a two-dimensional projection of cognitive state attributes aids their navigation through the space.MoodMixer is a collaborative experience that incorporatesaspects of both passive and active EEG sonification andperformance art. We discuss the technical design of the installation and place its collaborative sonification aestheticdesign within the context of existing EEG-based music andart.