  Spatial audio offers more immersive video consumption experiences to viewers;
however, creating and editing spatial audio often expensive and requires
specialized equipment and skills, posing a high barrier for amateur video
creators. We present MIMOSA, a human-AI co-creation tool that enables amateur
users to computationally generate and manipulate spatial audio effects. For a
video with only monaural or stereo audio, MIMOSA automatically grounds each
sound source to the corresponding sounding object in the visual scene and
enables users to further validate and fix the errors in the locations of
sounding objects. Users can also augment the spatial audio effect by flexibly
manipulating the sounding source positions and creatively customizing the audio
effect. The design of MIMOSA exemplifies a human-AI collaboration approach
that, instead of utilizing state-of art end-to-end "black-box" ML models, uses
a multistep pipeline that aligns its interpretable intermediate results with
the user's workflow. A lab user study with 15 participants demonstrates
MIMOSA's usability, usefulness, expressiveness, and capability in creating
immersive spatial audio effects in collaboration with users.
