from this site
what is meant by the italicized phrase? what other kind of sound is there? is it like the kind dogs hear, or something?
Perhaps it’s a reference to this story: Black Hole Sound Waves.
There couldn’t be sound waves of any kind in an absolute vacuum, but even outer space isn’t a perfect vacuum; there is dust and gas, in some places dense enough to be seen as nebulae, although even the gaudiest interstellar cloud is still a pretty good vacuum by Earthly standards. But even that very thin material can still carry sound waves, as detailed in the first article linked to.