Have you tried something like the following?
func _on_MasterSlider_value_changed(value):
settings_singleton.set_master_bus_volume_value(value)
AudioServer.set_bus_volume_db(AudioServer.get_bus_index("Master"),value)
If the audio settings aren't applied in other scenes, have them set on load:
# In the scene that requires changing the audio.
func _ready():
AudioServer.set_bus_volume_db(AudioServer.get_bus_index("Master"), settings_singleton.get_master_bus_volume_value())