In "The Solitary Reaper," why was the valley filled with music?

The valley is filled with music because a young woman is singing as she works harvesting grain.
The woman is all by herself in a lonely place in the Scottish Highlands when the speaker, passing by, stops to listen to her song. He can't make out the words, but the melody is plaintive and melancholy.
The young woman's song reminds the narrator of the song of a nightingale or a cuckoo, but it is more beautiful. The narrator states:

A voice so thrilling ne'er was heard

The narrator likens the woman to a natural creature as she bends and reaps and ties her bundles without ever stopping her song. He listens to her and comments that her song stays in his heart long afterwards.
The poem is quintessential Wordsworth because it celebrates an ordinary laboring woman, likens her to nature, and finds joy remembering the song long after he has finished hearing it.