2D shooter? Interactive sound sculpture? Why choose one when you can have both? Transcend
is an open-source, cross-platform (Mac/Windows build or *nix source), 2D shooter game. Thanks to morphing shapes and what the developer calls a "musical power-up system," as you shoot your way through the levels you're simultaneously "assembling an abstract visual collage and arranging a unique piece of music." With continuing advances in OpenGL 3D graphics and real-time sound, you can expect more alternative 3D interfaces for music like this one. (Thanks, Ronan!)
Unreal Tournament as Music Environment
More Unreal "Game" Performances.