Symbol Analysis
Music crops up a lot in this poem, but does so in a funny way. Namely, the poem systematically introduces music and then represses it.
The first time you see that is in stanza 3, where our hard-working Beadsman is freezing his bum off, praying for the sins of others and generally being wretched when—ta-da—he hears music coming from inside the castle. The music is so beautiful that it actually moves him to tears, and just when you think things are going to lighten up we get, "But no—already had his deathbell rung" (22), and then Keats goes on to talk more about the general awfulness of the Beadsman's lot in life.
In a poem that focuses on the truth (or not-truth) of the imagination, music cropping up and then being relentlessly shut down emphasizes the chasm between aesthetic desire and fulfillment. There are people who argue that the reason that Madeline and Porphyro peace out at the end of the poem is because, finally united, there's no room for them in a poem that focuses so much on the misplacement and frustration of desire.