Awakening up, he took her hollow 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 ear touching the melody;— Wherewith disturb'd, she utter'd a soft moan: He ceased—she panted quick—and suddenly Her blue affrayed eyes wide open shone: Upon his knees he sank, pale as smooth-sculptured stone.
Well, it doesn't look like Madeline's waking up, so Porphyro naturally decides to jam on her lute—because, you know, chicks dig musicians.
It's not any old song, either: Porphyro plays an old French ballad (Provence is in southern France).
That the song is "long since mute" most obviously means that it's super-old and hasn't been played in a really long time. Beyond that, though, it also includes the possibility that the song is still mute—that Porphyro is somehow playing a song that, in addition to being both "tumultuous" and "tenderest," is also both audible and silent.
Hey, wait, that song's title sounds… familiar. It's probably because Keats went off and wrote another poem entitled La Belle Dame Sans Merci in April of 1819, just a few months after he wrote "The Eve of St. Agnes."
Porphyro's totally weird, old song does the trick: Madeline's eyes open.
Check out the trade-off that happens in the last two lines: just as Madeline wakes up and comes to life, Porphyro sinks down, subdued and inanimate. And, hey, where was the last time we saw statues? That's right—back at the chapel with the "sculptur'd dead."