Space is a three dimensional region that contains everything that is not an atmosphere. In KSP, space is a complete void and has no effect on the motion of vessels. Space is broken up into spherical spheres of influence (SOI's) surrounding celestial bodies, which determine which body is effecting the craft via gravity, as KSP only applies 2-body physics (the body and the craft) when calculating trajectories and orbits.
When a spaceship goes very far (1.046*10^33 meters) all Celestial bodies and vessels will go to the origin of the three-dimensional space. There are no ambient sounds. An interesting thing is that, when you go very far, the letter of the altimeter will return to a m: at the beginning it's a m, then a k, a M, a G,..., a Y, a X and then back to a m when you reach 10^33 meters, or 10^17 light-years.