Star Wars - X-WING
https://www.youtube.com/watch?v=VWghOA692w0
A typical example is in the flight engine of X-Wing: While the player is flying, the engine plays the normal soundtrack, loosely based on Star Wars theme. When a Rebellion or Imperial craft enters the area, the music is interrupted and the brief 'Rebellion theme' or The Imperial March cue is heard.
Other cues contribute when a mission objective is reached or failed, when the player Hyperspaces back from a successful mission, or a failed mission etc.
The interruption of music, insertion of short cues and continuing of the music, uses a sophisticated algorithm which makes a smooth and almost unnoticeable transition. The result is heard like a unique, flowing piece, with its own variations that match the story.