Coriolanus is interested in the relationship between language and power. Shocking, right? Why on earth would a playwright who makes a living stringing together words have anything to say about the power of language? Some characters in the play (mostly aristocratic men and Volumnia) are really good at using words to get exactly what they want--they use language as a tool to manipulate others. Other characters (like the plebeians and Virgilia) have very little power and they lack strong voices. Coincidence? We think not. And that's what makes Coriolanus so interesting: he's got a strong voice, but he doesn't put it to good use.
Coriolanus' political failures might come from his unwillingness to keep his mouth shut, but at least he's a straight shooter who refuses to lie, flatter, and manipulate.
Despite the fact that the plebeians' votes are always referred to as "voices," the lower class characters have virtually no political voice at all in this play.