[danjayh]

The article implies that the test stand was running at a fixed frequency. This makes me suspect that this has to be designed to interfere with transmission of sound at a specific frequency, and perhaps isn't capable of eliminating more complex sounds like voices or music.

[lolc]

Yes. If you click through to the paper you can see the frequency response dipping around 460 Hz. Which is the frequency they're using in the demo.

[IshKebab]

Almost certainly. Most wave metamaterials aren't broadband. I can't actually remember any that are.