Symbol Analysis
As we discuss in "Wordplay: Contrasts," this poem swings back and forth between contrasts, and one of those is silence and sound. There are two interesting instances, however, where you seem to have music without sound.
The first is Keats's mention of the nightingale's song in stanza 23, when Madeline enters her bedroom:
As though a tongueless nightingale should swell
Her throat in vain, and die, heart-stifled, in her dell. (206-208)
As we mention in "Theme: Lies and Deceit," these lines are really ominous because they refer to the rape of Philomel, whose tongue was cut out by her rapist to prevent her from reporting the crime. Apart from the things it might say about the intentions of Porphyro, these lines are also talking about how Madeline is unable to express herself—there's something that's on the brink of expression but gets caught in her throat, "heart-stifled." This fits into the larger pattern we see with music: something bubbles up (whether it be the beautiful music coming from the castle hall, or whatever Madeline wants to express) and it's then shoved back down.