If you read the AudioStreamPlayer2D doc, it says it functions with the distance to the center of the screen. So I too would expect it to work as you intended, but if you read the Camera2D doc, you will see that its position (which is the one changing when following the player), does not represent the actual position of the screen.
I find quite odd that how this works, but I dont have a firm grasp of what goes behind a game engine, so who knows. I dont think I have a good solution for your problem, but you now at least know why it isnt working (the center of the screen does remain at (0,0), even when your camera moves)
You could try getting using
get_camera_screen_center() and then changing the screen center??? (dont know how haha)
maybe try with
Viewport.canvas_transform = get_camera_screen_center()
Hope it helps!!