Well, relativity is "on the way out" only on miniscule scales. Uncertainty messes around with the small measurements. But this has nothing to do with the universe being "flat" - that doesn't mean spacetime isn't curved, it means that the universe will keep on expanding and decelerating. Nothing to do with the relevance of relativity.
And the speed of light is fundamentally different from the speed of sound. We see plenty of things in nature that go faster than sound - astronomical objects, shock waves, heck, even light is an example. But nothing, NOTHING, goes faster than the speed of light through normal space (ie whatever the Universe is filled, or empty, with). Nothing can ever catch up with a photon, travelling the same path it took. But this doesn't mean that effectively FTL travel isn't possible - we could always try warping spacetime itself, to make the distances shorter, but then we lose the nifty time-shifting effects that this discussion group is all about.