Do other smooth surface spherical objects have the same effect?

Sound reflects off smooth surfaces. The ballon is probably just acting like any simple physical obstruction, because the surface does a lot all by itself even if theres almost no substance.

The air inside the ballon is also at a different density than outside, without helium, because of the elastic tension in the rubber. The air inside is always slightly compressed vs outside. I have no idea how much the two densities must differ to make the accoustic lense effect. I din't think it's this, just everyone seemed to be overlooking that even plain air will also have a different density.