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

Here's Harvard demonstrating it too: https://sciencedemonstrations.fas.harvard.edu/presentations/...

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.

Makes sense

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.

Oh dang! I bet that's it! Nice find, thank you!