In our world, Tin Pan Alley is the popular name for the area in New York City near 14th Street, where many publishers of popular songs had their offices in the late 19th / early 20th century. Aspiring composers would audition their new songs, and the din of so many songs being pounded out of pianos up and down the street gave the district its name. Another theory has it that the name derived from the rattling of tins by rivals when a performance was too loud and too protracted.
In England, Denmark Street, off Charing Cross Road, was also called Tin Pan Alley.
Today the phrase simply refers to the music publishing industry in general, and it is therefore no surprise that later, in Soul Music, we learn that the Guild of Musicians have their headquarters there.