This is a bit of a minor glitch, and it doesn't happen alot of the time, but when reliving the boss fights in Storymode, the background music will sometimes glitch out when skipping the end after defeating a boss. So, for example, if I'm fighting the Phaseman and I defeat him, it'll give me an option to skip the whole death scene. So I'll hold escape, and it'll skip it but the music that was playing before will still play. This will persist even when going through the portal and will follow you, even on the main menu screen. To fix it would just be restarting the game, and to date this has only happened to me twice and I've only been playing the game for about 5-6 days.