You can create a singleton scene called Sound.tscn which contains all your AudioStreamPlayers. A singleton is a scene that is always present as a node of your game, regardless of whatever other nodes you load.
You can specify a scene as a singleton in your Project Settings.
Once you have the singleton, elsewhere in your code you can make calls like this:
You can also create custom functions in a script on your Sound node which fade the music in and out, as well as transition between tracks.