Some games like Half-Life and Age of Empire II shipped with discs where the data was track 1 and the other tracks were the game soundtrack. You could put it in an audio CD player and it'd play it.
If you put another audio CD in the drive while playing, it'd play that instead of the game soundtrack. This was actually preserved for the Steam version of Half-Life, so there's posts on the internet of people were confused by it playing
Bob Marley or whatever.