Why is the speaker in "The Wild Iris" so concerned about "voice"? Well, speaking is a creative act, a way to express our innermost thoughts and feelings. Speaking is also a social act that allows us to form relationships. Some people even speak to deities, through prayer. For these reasons, it makes sense to think of speech as emanating from a source of inner power. Like a great fountain, our thoughts and feelings well up from this deep source and rise into an outpouring of spoken words. Or at least they do in this poem.
In "The Wild Iris," the ability to speak represents an individual's personal identity. So if you take away a person's voice, you take away their very self.
"The Wild Iris" suggests that the inability to express one's true thoughts and feelings can cause psychological damage, leading to a kind of emotional death. Yikes.