Re: Mach Numbers
Well I'm not the one who downvoted you - but I will answer your question (Knowledge after all should be shared and downvoting someone instead of explaining why they're wrong is just crass).
The speed of sound varies not only due to the medium (air, nitrogen, helium, you get the idea) but also due to the pressure and density of that medium. Near the ground on Earth for example, air is more dense meaning sound travels more slowly, 5 miles up it is less dense meaning sound travels more quickly. (the sound wave needs less energy to 'move' the less dense medium meaning it can travel more quickly). Even the temperature of the medium changes the speed of sound as a cooler medium is more dense and a hotter medium is less dense.
Given this, in space (which is not a complete vacuum) the speed of sound can be many, many times faster than the speed of sound at any height within Earths atmosphere.
Strangely the opposite is true for the speed of sound in liquids and solids, the more dense the liquid or solid, the faster sound travels.