To Althea, from Prison Art and Culture Quotes

Singing and poetry are often closely linked. Here, the speaker suggests that he will write a poem in celebration of his king, even though he is in prison. The rhyme on "king" and "sing" is interesting, for it suggests that there may be a close relationship between politics (the king) and art (song).

To "voice aloud" one's opinion implies that lots of people will hear it. It also seems to be a metaphor for publishing poetry, or anything for that matter. A "loud voice" can be heard by lots of people; lots of people will "hear," or read, the poem when it is published.