These are the voices which we hear in solitude, but they grow faint and inaudible as we enter to the world. Society everywhere is in conspiracy against the manhood of every one of its members. Which transcendental belief does this excerpt best illustrate?

The transcendental belief that this excerpt by Ralph Waldo Emerson illustrates is that society and government corrupt the individual.

The voices here refers to the voice of individuality, which we hear in solitude. Hence, as we enter the world the voices blend. Against the individuality, Emerson believes society to be "in a conspiracy." To make this more clear, Emerson makes an analogy with "Joint stock" of a company. He further refers, rather secure "liberty" it's better to eat. In additions refers that names and customs are its friends, not realities and creators.