Musée des Beaux Arts
What's interesting about our speaker is that he's pretty absent from the poem as a whole. No references to what he thinks or believes or ate for breakfast last Tuesday. No mention of his cats or his mother. In fact, the only way that we're able to learn anything at all about him is the fact that the poem's title tips us off to where he is: in the Musée de Beaux Arts. And that's not necessarily the speaker's doing. Come to think of it, the word "I" never once enters this poem. Which should tell you something. Whoever this speaker is, he's keeping a very low profile.
So why have a hidden speaker? Well, that's a good question. You could think of this guy as the voice in your ear when you rent an audio tour to the museum. He sounds authoritative precisely because he doesn't let on that he's a real person with petty thoughts and a sincere desire to get out of the art gallery and into the museum coffee shop as fast as he possibly can. In other words, he's the closest thing to a Voice from On High as you might get.
In a novel, he'd be a third-person narrative voice. In a Disney movie, he might just be a Conscience. In this poem, however, he's a speaker who doesn't ever move outside of a tight frame of reference. He's sitting before a single painting, and his mind is completely absorbed in what's before him.