Metamaterials have been used to make invisibility cloaks. To make an inaudibility cloak, all you need are some sheets of plastic and good mathematical modeling.
Sight is based on the assumption that light travels in straight lines. When we see something and want to get to it, we follow along a straight sightline. Most of the time that works just fine. Sometimes, like when the object is underwater or behind glass, we miss when we reach out to grab hold of something. This is because light bends when it hits the line between one medium and another. Our eyes can't see that the light has been diverted. We assume it came to us in a straight line from the object, follow it straight back, and find there's nothing there.
Invisibility cloaks, made of metamaterials, use our reliance on the orderly path of light to fool our sense. When light waves hit an object, the materials don't absorb or reflect them, they bend them. The waves travel around the object, continue their original course on the other side, and we assume that the light has come to our eyes uninterrupted.
The ability to gently bend waves, instead of breaking them, is not specific to light. Sound travels in waves as well, although waves of a very different sort. Sound waves are not electromagnetic. They're rhythmic waves of pressure traveling through the air. Objects absorb, reflect, and sometimes bend sound waves. To make something acoustically invisible, scientists have to construct a blind that would let incoming soundwaves proceed as if they had just scooted over a blank surface.
They managed to do just that by layering rigid sheets of ordinary plastic, each sheet filled with holes. When sound rushes through an opening, it spreads out on the other side in a circular pattern — the same way water waves spread out in a circular way when moving into a pool from a narrow channel.
When sound waves come through several holes at the same time, they interfere with each other. The high points become higher, the low points lower, and the high and low points cancel each other out. By running the sound through a number of these interference patterns, the stacks of plastic can bend it enough that — on the other side — it comes out like it never encountered any obstacle at all. Under this acoustic blind of stacked plastic, any object could be hidden.
The inaudibility cloak is limited for now. It can only hide an object around 10 centimeters long, and only from sounds with frequencies between 1 and 4 kilohertz. Unlike invisibility cloaks, it can be made with relatively simple products. All you need to be inaudible is plastic, patience, and smarts. The immediate use for this is concert halls, where lighting or other structures interfere with the acoustics. With time though, we may not be able to believe our ears or our eyes.