Sound (longitudinal compression waves) do indeed travel in vacuum, though inaudibly.
"Inaudible vibrations are tested by the magnetic needle and sound colours. Every gaseous molecule is a resonator of itself and is sensitive to any and all sounds induced, whether accordant or discordant. At the normal density of the atmosphere we hear a volume of sound, focalized by the combined association of every molecule brought under sound influence. When we reduce the atmospheric volume of a chamber to 50/100, then the ear is sensitive to the reduction of the acoustic force evolved on the same ratio, and so on, until sound becomes inaudible. This inaudibility to our organ of hearing is no proof whatever of any reduction of the acoustic force evolved on the introductory impulse given to the bell. It is only a proof that the number of the molecules left for the acoustic force to act upon has been so reduced by increasing the vacuum, that the concentration of sound from the diminished number cannot be heard. The ear is not susceptible to the acoustic force emanating from one molecule, nor even from the concentration of one hundred millions of billions molecules. The highest vacuum that can be induced, taking but a cubic inch in volume to act upon, will leave a residual number of molecules one hundred billion times as great as the above given number and yet be perfectly inaudible when all their acoustic force are focalized." [Vibratory Physics - The Connecting Link between Mind and Matter]
See Also
15.13 - Dissociating Water Acoustically - Liberation of Quantum Constituents
Ion acoustic waves in the solar wind
Vacuum