The audience of Tennyson’s “Ulysses” seems to change in each stanza. Why is this so?
I'm not convinced that the audience of Tennyson's "Ulysses" does change from stanza to stanza, merely that the focus of the poem shifts.
The poem is a dramatic monologue, spoken in the voice of Odysseus (Tennyson uses the Latin form "Ulysses") long after the time period of Homer's "Odyssey", when Odysseus, now an old man, has returned home to Ithaca and grown old. He is bored and restless, comparing the great deeds and adventures of his past with his current role of administering a poor remote island.
In the first stanza, the audience is not specified; it seems almost a soliloquy, in which Odysseus is talking to himself, but there is no evidence that the audience of the final stanza is not present. The second stanza begins with the statement "This is my son" which seems to indicate that someone else is present. In the third stanza, the audience is addressed as "you" and revealed to be the friends of Odysseus who have adventured with him in the past.
The progression of the poem is from the speaker trying to make a decision, to his actually deciding, and then finally to setting off. The audience becomes more fully integrated into the poem as it moves from reflection (an internal process) to action.