That ancient Beadsman heard the prelude soft; And so it chanc'd, for many a door was wide, From hurry to and fro. Soon, up aloft, The silver, snarling trumpets 'gan to chide: The level chambers, ready with their pride, Were glowing to receive a thousand guests: The carved angels, ever eager-eyed, Star'd where upon their heads the cornice rests, With hair blown back, and wings put cross-wise on their breasts.
Suddenly, in addition to the music, we have some sense of movement (people hurrying around have left the doors open), and the feel of the poem changes somewhat.
It sounds like these people are having quite the shindig ("A thousand guests"? Think of the bar bill.). There's so much coming and going that they can't keep the doors closed.
We've got more music, here, but it's got a different feel this time: instead of the "golden tongue" of yester-stanza, we now have "silver, snarling trumpets" which are "chiding." The music, which was all inviting and soft just moments ago, is getting a lot more aggressive.
Aaaand cue the lights.
Like in the chapel, there are statues here, but instead of the cold, dead, praying ancestors of the chapel, these are much more lively, "ever eager-eyed" angels, staring down ("Star'd" here is a poetic way of writing "stared") and carved with their hair blown back.
As you probably figured, the angels aren't actually staring and the music isn't actually "chiding" (teasing or correcting)—that's just Keats throwing down some handy-dandy anthropomorphism. While anthropomorphism generally just means "non-human things doing human stuff," here you specifically have artistic creations taking on human agency.
Let's find out if more of this is going on in the poem.