Viktor T. Toth :
It is often said that sound does not travel in space. And it is true … in empty space. Sound is pressure waves, that is, propagating changes in pressure. In the absence of pressure, there is no pressure waves, so there is no sound.
But space is not completely empty and not completely devoid of pressure. It carries sound.
For example, if you were to put a speaker in interstellar space, its diaphragm may be moving back and forth, but it would be extremely rare for it to hit even a single atom or molecule. It would, it would fail to transfer any noticeable sound energy to the thin interstellar medium. Even the somewhat denser interplanetary medium is too rare for sound to transfer efficiently from human scale objects; this is why astronauts can not yell to each other during spacewalks. And just as it is impossible to transfer normal sound energy to this medium, it does not necessarily transmit it, since its atoms and molecules are too far apart, and they just do not bounce into each other that often. Any "normal" sound is attenuated to nothingness.
However, if you were to make a million times bigger, and let's move a million times more slowly, it would be more difficult to transfer sound energy that thin medium. And that energy would propagate in the form of (tiny) changes in the (already very tiny) pressure of the interstellar medium, ie, it would be sound.
So yes, sound in the intergalactic, interstellar, interplanetary medium , and very, very low frequency sound (galaxies, solar systems) plays an important role in the formation of structures. In fact, this is the mechanism through which to make a comic energy and turn into something compact, such as a star.
How fast do you sound? Why, there is no set speed. The general rule for a so-called perfect fluid (a medium that is characterized by its density and pressure, but has no viscosity or stresses) is the ratio of the medium pressure to its energy density. The speed of sound, therefore, can be anything between 0 (for a pressureless medium, which does not carry sound) to the square root of three (for a very hot, so-called ultrarelativistic gas). 1