Symbol Analysis
A second, different example of soundless music comes from Porphyro. Unable to wake up Madeline, Porphyro takes the opportunity to break out some tunes on her lute. The song he plays, though, sounds kind of weird:
Awakening up, he took her hallow lute,—
Tumultuous,—and, in chords that tenderest be,
He play'd an ancient ditty, long since mute,
In Provence call'd, "La belle dame sans mercy":
Close to her hear touching the melody;— (289-293)
The song seems to be doing several contradictory things at once: it's both "tumultuous" (chaotic) and "tenderest" (soft), for instance. In addition, there's that word "mute." Now, we're told it's an old song ("La Belle Dame sans Mercy" is in fact a real medieval ballad), and so "long since mute" can be taken to simply mean that the song hasn't been played for a long, long time. However, there's also room to interpret these words as meaning that the song is still somehow mute, even as Porphyro's playing it.
At this point in the poem, we're pretty far down the rabbit hole with regards to telling what's real and what's imagined: Madeline's stuck in her "wakeful swoon," Porphyro's just woken up from his own dozing, and now he's playing a song that, in addition to being both "tumultuous" and "tenderest," also seems to be both audible and silent. Even the way the music is received by Madeline is weird. The melody is said to "touch" her ear; she's not hearing it so much as she's feeling it.
There's little that Keats digs more than describing sensory experiences. In the increasingly supernatural bedroom scene, Porphyro's "ancient ditty" focuses a lot of the weird stuff that's going on: the sensory synesthesia (mixing of senses) that crosswires both touch and hearing, and silence and sound, parallels the increasing ambiguity between dream and reality.