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As The Mountaintop, Katori Hall's debut Broadway play, begins, we hear the voice of Martin Luther King, Jr. accompanied by a trumpet, a lone horn singing with an elegiac yearning.

Those notes did not come easy.

Established musical-theatre composers like Andrew Lippa (The Addams Family) and Tom Kitt (Next to Normal) have added their touches to plays like The Farnsworth Invention (Lippa) and The Winter's Tale (Kitt). But now big names from beyond the theatre industry — Marsalis, Alicia Keys and Terence Blanchard — are writing what has traditionally been called "incidental music." (That term is tricky — the musician Stew, who recently wrote a score for A Midsummer Night's Dream, refused to be interviewed for this article because he found the phrase trivializing.)

"I've been thinking more about the feel of each play," says director Kenny Leon, who is working with Keys on Stick Fly after having recruited Marsalis for Fences and The Mountaintop. He decided to "throw the net wider" and pursue other types of musicians. "I think some of these composers haven't done theatre only because nobody had asked."

They may not be sitting home waiting for the phone to ring, but they are excited when it does. "I'm not seeking this out, but if the right person calls, I'm interested," says Lippa, for whom the right people were Farnsworth playwright Aaron Sorkin and director Des McAnuff.

Blanchard says that after he read the script of The Motherf**ker With the Hat, which producer Scott Rudin sent him, his reaction was, "Can I start right now?" He is also working on the forthcoming multiracial-cast revival of A Streetcar Named Desire starring Blair Underwood. Blanchard says he felt, as a New Orleans native and black musician, "it was extremely important and a huge honor to be part of it." He hopes to convey the city's flavor and culture, something he felt the movie failed to capture.