"To Althea, from Prison" is full of voices. There's the whisper of the first stanza, the speaker's claim that he will sing in a "shriller note," and his declaration to "voice aloud" his support for the king. There are also several references to birds, which are very vocal animals (they chirp and sing and sometimes even wake you up in the morning). The voices in this poem are all different. They range from the tender speech of lovers (whisper) to loud, boastful declarations of loyalty, but they're all tied into the speaker's ability to express himself, even from a jail cell.
Line 4: Althea is conducted to the speaker's cell so she can "whisper" to him. This act of the speaker's imagination suggests that his communicating must be secret.
Line 17: The "committed linnets" symbolize the speaker (who is himself also "committed," or locked up). As such, their song is symbolic of his poetry.
Line 18: The speaker says he will sing with a "shriller throat." The throat is associated with one's voice, and here—though the magic of metonymy—the speaker boasts of his ability to "sing," or write about, his king.
Line 21: The speaker will "voice aloud" his opinion that the king is good, and should be really great. "Voice aloud" is also a metaphor for publishing one's opinion, in this case the poem itself. The voice is "loud" because lots of people will "hear" it (by reading this poem).