Sometimes we only really remember one song in a musical. Of course there are exceptions. The Sound of Music, for example, is just filled with one memorable song after another.
But in many cases, one song stands out so thoroughly that it essentially becomes identified with the musical itself. It’s the show stopping number the audience waits for each night.
In such cases, when we see a poster for the musical, that song can start to play in our head. Some examples that come to mind:
Man of La Mancha The Impossible Dream Stop the World I Want to Get Off What Kind of Fool am I The Music Man Seventy Six Trombones Singin’ in the Rain Singin’ in the Rain The Wizard of Oz Somewhere Over the Rainbow Wicked Defying Gravity A Little Night Music Send In The Clowns Evita Don’t Cry For Me Argentina Cats Memory The Fantasticks Try To Remember Hello Dolly Hello Dolly
You can probably think of other examples.