This paper describes an application of constraint programming to interfaces for audio mixing. MidiSpace is an interface representing each sound source of a musical piece as a graphical icon, as well as an object corresponding to the listener in a window. MidiSpace is coupled to a spatialization system so that moving graphical objects modifies the audio mixing of the musical piece according to the respective position of the sound sources to the avatar. We further introduce a constraint-based mechanism which allows to maintain consistency in the overall mixing. Constraints represent properties of related sound sources, which should always remain true, and may be stated by the user through the interface. When an object is moved, a constraint solver uses the constraints to propagate changes. We describe the library of currently designed constraints, and propose an extension of the system to handle reproduction systems with multiple loudspeakers.