You could create a scene that has the music node , and then make a singleton(autoload) of it.
Controlling it would be just a matter of calling its functions or variables from any other script of your game.
It would be loaded globally and you'd be able to play or pause songs that would play between scenes.
Have a look at godot's singleton page
Looks like your explanation was a little misleading for me, so now that I understood:
You should use "SamplePlayer" instead of SamplePlayer2D.
The main difference between the two is exactly this: SamplePlayer plays globaly on a scene, with no regards to the camera or viewport.