The court might rule in favor of his estate for this reason. But honestly, I do think there are differences to a singer (whose voice becomes an instrument in their song) and a comedian (whose voice is used to communicate the ideas and jokes they want to tell). A different voice could tell the same jokes as Carlin, and if done with the same level of care to communicate his emotions and cadence, could effectively create the same feeling as we know it. A song could literally be a different song if you swap an instrument. But the courts will have to rule.