It’s a bit oversimplified, actually. Sound bounces off of discontinuities in the medium, which is why foam works. You just have to control scattering somehow.
The big problem with using oobleck or whatever is it responds to shear, and shear can’t travel through air. You could use it for earthquake protection, though, or if you could channel compressive waves from the air into shear form using a fancy bridge like in OP.