My intuition is that ideal gasses must have some form of quantized quasiparticle/collective excitation (I'll just call it a quasiparticle), but it usually isn't (or never is) a useful concept. (Just like a basketball at room temperature has quantized energy levels, but it's not useful to think of them.) Whether you call that quasiparticle a "phonon" is a matter of semantics. Quantized sound waves in liquids are called phonons, so I guess quantized sound waves in gasses could be called phonons too.

So here are some random thoughts:

Remember that what we think of as a conduction electron in a solid is really a quasiparticle involving the other electrons and ions in the system; a conduction electron is not simply one free electron.

Likewise, I guess that at some level an atom in a gas is really a quasiparticle shaped by the other atoms in the gas. The difference between conduction electrons in solids and atoms in gases is that the latter interact much more weakly, so an atom quasiparticle (at room temperature) will look almost exactly like a lone atom. That means the quasiparitcle picture isn't useful, but it isn't wrong either (like the quantized basketball).

While, at room temperature, that atom quasiparticle will look nothing like a phonon, my guess is that if the thermal energy of the atoms in the gas is similar to the "binding" energy between the atoms in the gas, then the atom quasiparticle could start to look like a phonon as we normally think of it (or at least look like a phonon in a liquid). That said, if the thermal energy is comparable to the "binding" energy, then you probably won't have a gas any more; it'll probably be in another phase (solid, liquid, Bose-Einstein condensate, etc.). Maybe that means that the concept of a gas phonon isn't wrong, but it is never useful either.

My intuition is that ideal gasses must have some form of quantized quasiparticle/collective excitation (I'll just call it a quasiparticle), but it usually isn't (or never is) a useful concept. (Just like a basketball at room temperature has quantized energy levels, but it's not useful to think of them.) Whether you call that quasiparticle a "phonon" is a matter of semantics. Quantized sound waves in liquids are called phonons, so I guess quantized sound waves in gasses could be called phonons too.

So here are some random thoughts:

What we think of as a conduction electron in a solid is really a quasiparticle involving the other electrons and ions in the system; a conduction electron is not simply one free electron.

Likewise, I guess that at some level an atom in a gas is really a quasiparticle shaped by the other atoms in the gas. The difference between conduction electrons in solids and atoms in gases is that the latter interact much more weakly, so an atom quasiparticle (at room temperature) will look almost exactly like a lone atom. That means the quasiparitcle picture isn't useful, but it isn't wrong either (like the quantized basketball).

While, at room temperature, that atom quasiparticle will look nothing like a phonon, my guess is that if the thermal energy of the atoms in the gas is similar to the "binding" energy between the atoms in the gas, then the atom quasiparticle could start to look like a phonon as we normally think of it (or at least look like a phonon in a liquid). That said, if the thermal energy is comparable to the "binding" energy, then you probably won't have a gas any more; it'll probably be in another phase (solid, liquid, Bose-Einstein condensate, etc.). Maybe that means that the concept of a gas phonon isn't wrong, but it is never useful either.