Create a scene which is just an audiostreamplayer with the music you want. You may want to set it too autoplay.
The go into project > project settings. In the tabs you will see autoload. Go to the autoload screen and you can add any node to this, what this will do is automatically load the scene you add whenever you load any other scene, so it is the equivalent of adding it to each scene you load in the game. This should mean that your music will keep playing between scenes.