Fire and Hemlock by Dianna Wynne Jones (a kind of combo of the Tam Lin ballad and Thomas the Rhymer)

Winter Rose by Patricia McKillip (another Tam Lin reference)

Thomas the Rhymer by Ellen Kushner

Imaginary Lands -- ed. by Robin McKinley (short story in there about Tam Lin)

Song for the Basilisk by Patricia McKillip (all about music and its power)
Art and Lies by Jeanette Winterson (not totally fantasy, more like magical realism, an adult book, it ends with a piece of music and music and singing us vital to the plot)

The Horns of Elfland ed. by Ellen Kushner (all about faery and music)

Sister Light, Sister Dark by Jane Yolen (not so much about music but certainly depends on the tradition of ballads and their place in the historical record)

High Fidelity by Nick Hornby.

Delia Sherman's Through a Brazen Mirror: the Famous Flower of Servingmen.