[quantisan]

We had the same idea at a hackday and built it. https://github.com/Quantisan/nullwave

We used the laptop's mic as the source of environmental sound and just flipped the signals and play them back on a headphone. Because of the distance between the laptop and our ears, the signals can't really be cancelled that trivially.

The app ended up amplifying non-noises (we had some filtering) and we could hear other people talking quietly even across a room. So we jokingly said we built a spying tool on demo.