In the Mario platformers, who's to say that the blocks are made out of brick? Just because they have a brick-like appearance, doesn't mean they couldn't be made of something like polystyrene.
Also, in Mario and Sonic at the Olympic Games, Mario and Sonic are from different universes, where the physics may be different to each other. The resistant forces in the Mario universe may be incredibly strong, preventing him from travelling very fast, and the resistant forces in the Sonic universe may be incredibly weak, allowing him to travel quickly. Therefore, when they meet in our universe, to compete in the Olympic's, the resistant forces would be at a balance between the two other universes, meaning that the resistant forces would be weaker than those in Mario's universe, allowing him to travel faster, and they would be stronger than those in Sonic's universe, preventing him from travelling as quickly as he normally would be able to. This would put they're speeds at an equilibrium, meaning they can both move as fast as each other.
So there, both of your statements have been disproved.
~Peace Out,
~Jiggi.
P.S. Sonic cannot break the sound barrier, he runs just underneath it. He can only break the sound barrier if he uses his Light Dash ability, which can only be used along a trail of rings.