"Voice" is a literary element that is not the mood, or the author's tone. It is, rather, the persona created in the writing—the poem—in which the author has developed the poem's voice. It should not be confused with the author's ideas or feelings, which would be the tone. Here are two definitions:
Voice is associated with the basic vision of a writer, her general attitude toward the world.
It is a convention in poetry that the speaker is not the same individual as the historical author of the poem.
The voice Adrienne Rich uses in her poem, "Song," is one of self-awareness on the part of the speaker.
So for Adrienne Rich's poem, "Song," the voice I "hear" is that of a woman who seems to have been told that she is lonely. (Remember, this is not the author speaking, but the woman she has created on paper.) The descriptions the speaker presents may serve as examples for her listener to the contrary: loneliness is not a part of this woman.
The speaker is challenging the concept of loneliness, and she is positive in the presentation of her "defense."
The speaker defines what loneliness is NOT, in her life. She speaks of flying in a plane across the Rocky Mountains toward an ocean.
She describes a woman driving through towns in which she chooses not to stop and stay, knowing that if she did, then she would be lonely.
The speaker draws upon the image of waking early to a slumbering house, and snuggling in the comfort of a place "wrapped in sleep."
Finally, the speaker presents the loneliest picture of all: a row boat entrenched in the ice on a shore. Through personification, Rich tells the reader that in this solemn situation, the old boat knows, as the speaker of the poem does, what it is and what it is not, and despite the impression of defeat or loss, the boat understands that when all else fails, there is still hope: the boat can provide the "gift" of burning—light, life and warmth.
The speaker tells the reader that unless the concept of loneliness has changed, she is not lonely. At the worst of times, there is something living in her that can dispel darkness, fear or desolation: she has a fire within.
In Claude McKay's poem, "America," the "voice" is of determination to survive, and the deliberate dedication to the very thing that threatens the speaker.
In the first section, we hear the threat of "America" with words and phrases such as: "bread of bitterness," "sinks into my throat," and "stealing my breath of life."
The shift to the speaker's dedication to American stands out against the threat:
"I love this...hell...that tests my youth."
And the reasons for that continuing "love," go on:
"Her vigor flows into my blood, / Giving me strength...against her hate."
The speaker's dedication of purpose is felt in:
Yet as a rebel fronts a king...I stand within her walls with not a shred / of terror, malice, not a word of jeer.
McKay may speak to prejudice and violence against people of color. He may speak, too, of poverty which engulfed the US during the Great Depression of 1929-1939, and beyond.
The voice is that of a newcomer who embraces a land with all its trials and tribulations, along with its beauty and promise.
The voice in both poems speaks to the determination to embrace that which defines each speaker, even in the face of adversity, from wherever it may come. Both speakers are determined to find hope in his/her circumstances, and not be defined by what each encounters.