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I think it becomes around the time when the mean free path of a molecule of the gas is the same order of distance as the wavelength of the sound you are trying to transmit (in other words, ultra low frequency sounds may be transmitted even in a near vacuum).
I bow to your expertise, but can you hear, for example an exploding spaceship in space knowing that there are gases, like oxygen, in the ship at point of explosion. Is it possible for the oxygen and other gases to be thrown clear so that the sound can also be transmitted to places other than the spaceship?Space movies sometimes puzzle me in this regard because you can clearly hear the engine of a spaceship humming as it travels along...
In an ideal gas the gas pressure shouldn't change the speed of sound
The limitations of the concept of speed of sound due to extreme attenuation are also of concern. The attenuation which exists at sea level for high frequencies applies to successively lower frequencies as atmospheric pressure decreases, or as the mean free path increases. For this reason, the concept of speed of sound (except for frequencies approaching zero) progressively loses its range of applicability at high altitudes.
Greenspan  described this behavior in the follow-ing way. "Nonuniform effects in gases are best studied at small acoustic amplitudes where relaxation effects can be observed. From kinetic-theoretical considerations in a gas of smooth rigid spheres, the speed of sound depends only on the mean speed of the molecules, provided that the gas is sufficiently dilute so that practically all of the molecular momentum is transferred and that the mean collision rate is very high. Laplace's formula states that the sound speed depends on the molecular mass and the temperature, and these deter-mine the molecular speed. Any dispersion must depend on the ratio of collision rate to sound frequency. A suitable parameter for comparison is one proportional to this ratio (pressure/ frequency). For example, a sound wave at an audio frequency of some kHz in a gas near atmospheric temperature and pressures will have a mean collision rate of order of 10 10 s Ð1 . The medium is very fine grain and the dispersion is negligible. As the sound frequency is steadily increased, the frequency becomes comparable at first to the collision rate of the slower molecules. The collision rates becomes positive corre-lated with the molecular speed. The slower molecules can not transfer the acoustic momentum coherently, and this burden shifts to the faster molecules. Accordingly, the speed of sound steadily increases with frequency. The effect is negligible unless the frequency is very high."This work was extended to the measurement of viscosity effects for the diatomic gases nitrogen and oxygen and to dry air. Moe compared his theoretical predictions  to the attenuation and propagation constant of the diatomic gases as measured. The behav-ior at higher pressures follows the classic theory. As the mean free path approaches the ultrasonic wave length, Greenspan's constructed theory fitted at the intermedi-ate pressures. For lower pressures significant deviations were found His work on propagation of sound in rarefied gases is a classic example of how to examine a complex system for relaxations resulting in a measured dispersion. He was able to show that the Stokes-Navier equation gave a surprisingly good quantitative account of attenuation and dispersion of sound in monatomic gases down to wavelengths approaching the mean free path. Moreover, he succeeded in making measurements to much lower pressures where the mean free path was significantly greater than the wavelength, and found substantial deviations from the theories. New theoretical many-body results are now judged by their agreement with these data. For diatomic and polyatomic gases, where molecular relaxation processes associated with vibra-tional and rotational modes occur in addition to the translation relaxation, he was able to demonstrate experimentally and theoretically how they combine to affect acoustic dispersion and attenuation.