[dguest]

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.