Depends on the specifics. You can do that with waves. The way noise cancelling headphones work is exactly this. They just emit the opposite waves of the noise around you into the headphone. Pretty smart.
>expect sign of intelligence
Technically she wouldn't be able to produce a simple wave to cancel out the other as human vocal chords produce complex waves (two or more overlapping waves) but I assume her power just ignores this, making it entirely plausible.
It would only work if Robot and Koito emit the sound in the same direction with the phase shift as they show in the next picture.
These two waves would not cancel out each other since they are going in opposite directions without being symmetric thanks to the increasing amplitudes.