In the first videos he was in, he sounded like a stupid noob, but as time progressed, he went from that, to an epic pro voice, to the accent we know today. Bonnie The Bunny and Toy Bonnie are cousins
It wasn't until the ninth episode of FNAF World that he got his current voice, sounding like Bonnie's voice, but higher pitched (and says "Fwah" as fowa). However, his other two voice are apparently stuck in his head and will start to talk at random moments. It's currently unknown why this is. It is presumed this happened because he entered the "world between worlds" in FNAF World. Either that, or he's going insane (God save the queen am I right lads?). He keeps this fact a secret to the others most likely to not be presumed to be insane.
As of "How Will Toy Bonnie and Toy Chica Work in Ultimate Custom Night?", it has been revealed that everyone else already knew he had 2 voices in his head. The Pro and Noob voices have since taken residence in RockStar Bonnie and El Chip respectively.