Music sung or played at night below a person's window

One word substitute is Serenade.

Serenade: a piece of music sung or played in the open air, typically by a man at night under the window of his beloved.
Sonnet: a poem of fourteen lines using any of a number of formal rhyme schemes, in English typically having ten syllables per line.
Lyric: (of poetry) expressing the writer's emotions, usually briefly and in stanzas or recognized forms.
Primo: the leading or upper part in a duet.