it maybe acting as an acoustic lens.
https://www.youtube.com/watch?v=RLBmWF9Xo10
edit: Steve Mould's video "I Made a Lens, But for Sound" demonstrates how balloons filled with gasses of different density than the surrounding air, act as a lens on sound waves. Helium filled balloons will scatter sound because the helium is less dense than air. He shows how a balloon filled with carbon dioxide can focus the sound.
Somebody wrote a paper on it in 2008: https://physics.byu.edu/docs/publication/644
A balloon filled with a gas that has a different sound speed than that of air has been used as an
acoustic lens. One purpose of the lens is to show refraction of sound waves in an analogy to
geometric optics. We discuss the physics of the balloon lens demonstration. To determine the
validity of a gas-filled balloon as a classroom demonstration of an acoustic lens and to understand
the corresponding phenomena, its physics is considered analytically, numerically, and
experimentally. Our results show that although a geometric analogy is a good first-order
approximation, scattering theory is required to fully understand the observed phenomena. Thus this
demonstration can be adapted to a wide range of students, from those learning the basic principles
of refraction to advanced students studying scattering
So would a wall of helium balloons work as a sound dampener? Or perhaps diffuser? A lot cheaper than fiberboard & foam
I doubt it would be cheaper than foam, but this is similar to gas filled windows. Argon or Krypton gas is pumped in-between the window panes to provide another layer of insulation.
At the old Exploratorium in the Palace of Fine Arts there was an exhibit that had a large 3-4m balloon filled with something heavy (Argon or maybe SF6?) and two points on the floor at the foci of the balloon. You could whisper at one focus and hear it easily at the other. I think it has been replaced with a more durable pair of concrete parabolic reflectors with similar effects.