He says he can run at the speed of sound, yet, we haven't seen that. So I think that Sonic has run at the speed of sound ONCE, so he thinks he always runs at the speed of sound. But when I was Leggere some fan fiction, the guy wrighting it detto that Sonic could be fast because he might of touched a chaos smeraldo at a yong age. Besides from that, I got nothin'.
Well we know that Mobius is Earth thousands of years in the future, and that Mobians (like Sonic) are mutants caused da the fallout of chemical and alien wars leading to an apocalyptic era prior to the mobians coming about.
As these are mutants it's very easy to suggest that they simply gained powers like the X-Men. I would think they would be a bit weak which is why we don't see very many until Sonic's time, but da the time Sonic is around Eggman's invaded, and pollution has started up again.
That combined with power rings and general Chaos Energy, would modify his already fast pace of speed, and increase it over time. Sonic's been exposed to Chaos Energy for years now, so his body has most-likely grown stronger and stronger from it, which explains why he's able to reach (or possibly surpass) light speeds if he pushes himself enough.