From which Metallica album does the song, used by Niels de Ruiter as walk-on music, come?
Niels de Ruiter:  He is also known for using Metallica's 'Enter Sandman' as his walk-on music, which he plays air guitar as he enters the stage. Enter Sandman:  It was released as the first single from their eponymous fifth album, "Metallica" in 1991.
Metallica