I was doing some research online about slower-than-light interstellar spacecraft, and it got me thinking about a couple things. Although it may seem empty, the space between stars isn't a perfect vacuum. There's still dust, stellar wind, gas clouds, and many other particles floating around that make up the "interstellar medium". Although its effects are negligible for ordinary spacecraft, a ship traveling a good portion of the speed of light would start to feel "atmospheric" effects from all those particles running into it. So, is it possible to generate a supersonic shock wave if a ship travels fast enough?
There is no such thing in space
The method of propagation of the speed of sound in substances differs by their physical state; in space, the speed of sound would propagate as in a plasma. In outer space with few particles, a particle deflected by interaction with a spacecraft has a relatively low chance of hitting another particle.
The shockwave of breaking the sound barrier can be envisioned as a pressure increase caused by particles stacking up against the moving object. But, in space, there simply aren't many other particles around. There is no pressure buildup, because there is plenty of space near a passing spaceship that a particle can relocate to without bumping into other particles.
So, nearly anything that is moving in space is already "breaking the sound barrier". Since we don't see the kind of supersonic shock waves you are expecting around satellites (or the moon), we won't see anything like that by going near to the speed of light.
You can have shock waves in the solar wind.
The solar wind is comprised of particles streaming away from the sun. As in your OP, one could consider this as a medium similar to our atmosphere - although much less dense and moving much faster. A fast moving thing plowing into this medium could generate the equivalent of a shock wave.
... These shock waves occur when a fast-moving pulse of solar wind particles plows into the rest of the slower-moving solar wind. Such a pulse can be created by a solar flare or other kind of solar eruption, for example. Particles in “fast” solar wind can travel up to two times faster than those in “slow” wind, which still zips along at speeds more than 1000 times faster than the speed of sound in air. The wave crest, where fast ions hit slower ones, is a compressed band of wind that has increased pressure, density, and temperature compared to the surrounding solar wind.
In the example, the fast thing generating the shock wave is faster solar wind, but it could be a ship - especially if it were a big ship or for some other reason had its zone of influence sweeping out a large swath of space and particles. It is not a supersonic shockwave because sonic means sound. But the analogous shock wave in the solar wind is similar to a shock wave produced in the atmosphere on earth. If you wanted this sort of thing to happen in your story it would be totally plausible.