Once your screenplay has been sold, do you (the writer) have any say in the music that is used?
Almost never. The music in a movie is ultimately the decision of the director and a contentious committee that includes the composer, the music supervisor, the editor, the producer, the studio, and the soundtrack representative. Sometimes the writer will be included in that group, but rarely, because the problem is usually too many opinions, not too few. An exception is when a very specific song needs to be used for a scene. For example, in CHARLIE’S ANGELS, a scene opens with Tom Green’s character singing "Angel of Morning" while making breakfast, and that was always in the script.