This happens to me often, especially when on "autopilot" and not having to push entirely all out or when nav task is not that challenging. I can't select or change song just like that and I have no idea where the song comes from. Last weekend in our national champs middle qualification race the song was "Pulp Fiction" https://www.youtube.com/watch?v=ImyU654znR8
In the final there was no songs, just pain instead for trying to go all-out all the way. It is not a much problem really for "autopilot" (old habits take over, doing about right things and never entirely wrong things, without having to think too much about it) and for song disappearing by itself when there is something challenging at hand to focus to.