This is to do with non-gaming characters and non-SEGA characters, that be to give them voice acting which they don't have probably for copyright reasons, even tough other video games have been able to provide voice acting for characters not owned by the publisher of them no problem, most notably with non-Nintendo characters in the Smash Bros. franchise that of course includes Sonic: https://www.change.org/p/urge-sega-to-add-voices-to-dlc-characters-in-sonic-racing