"The Third Man" theme played on a zither by Anton Karas

""The Third Man Theme"... is an instrumental written and performed by Anton Karas for the soundtrack to the 1949 film The Third Man. Upon release the theme proved popular, spending eleven weeks at number one on Billboard's United States Best Sellers in Stores chart. Multiple versions have been performed and recorded, selling tens of millions of copies, and its success influenced the release strategy of later film singles.

The Third Man is a 1949 British film noir, directed by Carol Reed. One night after a long day of filming The Third Man on location in Vienna, Reed and cast members Joseph Cotten, Alida Valli and Orson Welles had dinner and retired to a wine cellar. In the bistro, which retained the atmosphere of the pre-war days, they heard the zither music of Anton Karas, a 40-year-old musician who was playing there just for the tips. Reed immediately realized that this was the music he wanted for his film..."

Here's the trailer for the movie -