The Future Isn't Invisible, But It's Real Quiet
In order for a material to work as an acoustic cloak, the speed of sound passing through it must be direction dependent. That is, sound waves traveling through the shielding material from one direction must move at a different speed than waves traveling in a perpendicular direction. These differences create scattering effects that should direct sound waves to flow over a shielded object like water flowing around a rock. Because the waves return to their original conformation after passing such a shielded object, the object effectively becomes invisible to sonar. And a listener inside such a shield wouldn't hear the sounds flowing around.
The scientists have demonstrated the idea, but there are technical difficulties with the engineering of the substance itself that must be overcome before you'll be getting any off the shelf at Wal-Mart.