The speaker is busy trying to write a duo, a piece of music for two, in a land soaked with rain. The word "compose" suggests careful, deliberate work, and the title "Solo" hints at the irony: the speaker is attempting something for two while being alone. Then a sound interrupts. Someone is singing, and the speaker wonders who it could be.
What stands out is the contrast. The speaker is striving, while the unseen singer simply sings "with so much pleasure / to herself." Nothing is being produced or performed for anyone. The song seems to exist only for its own joy, and that is exactly what the speaker, mid-effort, may be missing.
The mood is quiet, curious, and a little wistful. "Rain-drunk" is a lovely phrase, making the whole landscape feel loose and happy, as if the singer belongs to it. The poem ends on a question, leaving us listening along with the speaker, charmed and perhaps slightly envious.