This is a common consumer product, "gas fill" multi-pane windows. Argon is used in this application because it's not very thermally conductive (it's also used to fill dry suits for the same reason). I found a pretty good "sciencey layman" explanation of "why Argon" for dry suits here, which matches the reasoning for windows: https://www.decompression.org/maiken/Why_Argon.htm .
I'm not sure how much this carries through to sound transmission and I couldn't find a lot of good literature about it. I'd have loved to see if they did any quantitive testing. It makes sense that Argon would reduce sound transmission some, but I would expect that the properties of the glass sheet itself and the interface between the panes and the frame would be more of an issue than the void between the panes.
The insulation part has a nice handwaving explanation [1]:
Heat is transmitted because gas molecules bounce back and forth in a box, picking up energy on the hot side and leaving it on the cold side. The kinetic energy in any gas molecule is proportional to the temperature, and is
E = 1/2 m * v^2
solved for v:
v = sqrt[ (2 * E) / m ]
Faster gas means faster transmission, so the rate is proportional to 1 / sqrt(m).
I don't have a good intuitive explanation for the sound attenuation. The acoustic impedance for an ideal gas is going to depend on the mass of the molecules, and having very different acoustic impedance for the two gasses at an interface will minimize transmission. So I would expect Argon to reflect more sound, but I don't have as cute an explanation as the one for thermal transmission.
[1] Also completely made up, would love to know if I'm wrong.
I'm not sure how much this carries through to sound transmission and I couldn't find a lot of good literature about it. I'd have loved to see if they did any quantitive testing. It makes sense that Argon would reduce sound transmission some, but I would expect that the properties of the glass sheet itself and the interface between the panes and the frame would be more of an issue than the void between the panes.