Matt Marks, a composer and musician who was at the epicenter of a diverse community of open-minded artists as a founding member of the contemporary chamber orchestra Alarm Will Sound, died on May 11 in St. Louis. He was 38.

The cause was heart failure, said Mary Kouyoumdjian, a composer and Mr. Marks’s fiancée.

Mr. Marks, who lived in Brooklyn, had just performed in St. Louis with Alarm Will Sound, she said.

As a performer, Mr. Marks was known best as a French horn player for Alarm Will Sound, of which he was an integral member. The ensemble has been critically praised and is known for its unusual stylistic breadth and commitment to innovation.

When the group ventured further into theatrical concerts and multimedia events, he rose to the occasion as a singer, an actor and a keyboardist. He also contributed one of the group’s signature pieces: an eerily accurate arrangement for live performers of “Revolution 9,” the notorious 1968 musique concrète sound collage recorded by the Beatles.